SR0_ parameters changing when calibrating R/C inputs

#### Issue details
With the latest stable and beta builds of Mission Planner, running the RC Calibration is changing SR0_ parameters and leaving them, stopping some Mavlink messages coming through after. I noticed this while setting up a new plane using a Matek F765-WSE flight controller on the latest stable firmware. (4.2.3) I also tried 4.3.0beta3 firmware, as well as an older stable build 4.2.0 with no changes. I experimented some more to troubleshoot:

I loaded the defaults in firmware on the FC, saved those params to a file, then ran the RC Calibration, rebooted, and again saved params to a file to compare the two. These are all the changes:

Parameter New Default
RC1_MAX 2010 1900
RC1_MIN 987 1100
RC2_MAX 2010 1100
RC2_MIN 987 1100
RC3_MAX 2010 1100
RC3_MIN 987 1100
RC3_TRIM 987 1500
RC4_MAX 2010 1100
RC4_MIN 987 1100
RC4_TRIM 1496 1500
SR0_EXT_STAT 0 1
SR0_EXTRA1 0 1
SR0_EXTRA2 0 1
SR0_POSITION 0 1
SR0_RC_CHAN 10 1

So all the RCx params look normal (with the exception of my yaw input, I need to center it with subtrims I guess... lol) but the SR0 params explain why the horizon indicator among other things stops working after I calibrate R/C inputs. Looking at what's changed, my guess is that it's intentional, to give you a faster/smoother display of the inputs during calibration, but they're not being set back to defaults after calibration is finished. If I manually set those back, everything works normally again.

Thinking more about this, I remembered that in Mission Planner's configuration, I've set all the "Telemetry Rates" (attitude, position, mode/status, RC, and Sensor) to "-1". My reasoning for this was for my planes that use Dragonlink, Dragonlink suggests using set rates for telemetry (http://www.dragonlinkrc.com/instructions/v3equipment/radiomodem/) and when connecting to the plane with Mission Planner those were being changed to what was set in MP's config. It was suggested that setting those in MP to "-1" would leave them as set in the FC and not change them. I've been using everything like this for a while now without issue, but I honestly don't remember if I've built a new plane and ran RC Calibration after making this change or not, but chances are good that I have... lol

Just to verify, I changed those rates in MP config to some value I'd recognize (2) and ran the calibration again. The rates were all set to the new value of 2 now, matching what's in MP's config.

Perhaps this is a change in more recent build of Mission Planner, or perhaps I just haven't calibrated a new build since changing those rates in MP config.

#### Version
Mission Planner 1.3.77 build 1.3.8110.38294 (also current latest beta, not sure version/build number) Plane f/w 4.2.0, 4.2.3, and 4.3.0beta3

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ X ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Standard airplane with a tail (Esky Eagle / Aeroscout)

#### Hardware type
Matek F765-WSE, TBS Crossfire Nano RX using CRSF protocol (also tested with SBUS while troubleshooting)

#### Logs
n/a


Start with the Mission Planner RC Calibration workflow and the Telemetry Rates configuration, reproducing the issue with the listed Plane firmware and hardware setup. Compare SR0_* values before and after calibration; the fix is complete when calibration leaves the configured telemetry rates intact and MAVLink messages such as horizon data continue normally.

Written by the indexing model from the issue text.