Fix ESCs constantly arming on rover with dshot commands

[andyp1per]

Several bugs fixed:

• ESCs constantly arming on rover with dshot commands
• Bdshot not working with fewer channels than the full group
• Not disabling disabled channels
• Not pulling high all channels in a bdshot group (caused random motor movements in disabled channels)
• Not ignoring disabled channels for telemetry starting position

Although this affects all vehicles it will particularly help rover and plane

[andyp1per]

@rmackay9 this appears to fix all of the bugs as far as I can tell including LED output.

[rmackay9]

I've tested this on a pixracer and CubeOrange (using the special bdshot firmware for Rover) and confirmed that the motor spins correctly and I'm able to get RPM from the ESC using bi-directional DSHOT but only when the throttle is on the first fmu channel. The motor does not spin if the throttle is on the 2nd FMU channel.

If I used this PR's code with bdshot disabled it all worked OK even on the 2nd channel.

With master and BDshot enabled the ESC do not arm reliably and when they do they are often followed by some pretty scary motor movements.

[andyp1per]

Thanks @rmackay9 should be fixed now. There were actually two remaining bugs, both of which were extremely subtle.

[andyp1per]

Flown on a miniquad without issue - bi-directional dshot all ok.

[rmackay9]

I've tested this and it resolves the issues I was seeing on Rover, thanks very much!

[rmackay9]

This has been added to Copter-4.2.0-beta3