RC_SPEED = 490 Hz default param (and it isnt really)

#### Issue details
Why is RC_SPEED defaulted to 490Hz in copter? Capturing motor pin outs with an oscilloscope results in 100-400Hz rates being correct, however above 400Hz isnt a thing.

We did notice some odd "jitter" with 490Hz settings, however have been unable to reliably recreate it.
#### Version
3.x

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

#### Airframe type
Quad

#### Hardware type
Cube 2.1

Start by locating the RC_SPEED parameter definition in Copter and compare its configured value with the oscilloscope measurements described. Determine whether the default and observed output rate disagree, and investigate the reported jitter; done means the behavior and default rationale are reproducibly explained or a specific correction is identified.

Written by the indexing model from the issue text.