hzratecheck hard coded tests.

#### Issue details
The implementation of hzratecheck() in MAVLinkInterface.cs consists of a set of tests against magic numbers (which do not match those presented in the UI), such that for some requested data rates, the check always fails, resulting in the request message being repeatedly issued.

Further, the checks for 3Hz and 10Hz both contain an obvious typo.

#### Version
Master

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

#### Airframe type
N/A

#### Hardware type
N/A

#### Logs
N/A

Read hzratecheck() in MAVLinkInterface.cs and compare its hard-coded rate checks with the rates presented in the UI. Done means the checks match the requested rates, including correcting the noted typos, so valid requests are not repeatedly issued.

Written by the indexing model from the issue text.