Transmitter Control: only set AltHoldDesired when exists

Because AltitudeHoldDesired is only initialized when the module
is running (which applies at startup) but the flight mode
position can be changed on the fly, this can allow the FC to
write to an object that doesn't exist and then crash.

Fixes #865
ManualControl: set alarm if failing to set AH

Provide a rich alarm if the user is attempting to enable AH
but the module is not available.
ManualControl: set alarm if failing to set PH

Provide a rich alarm if the user is attempting to enable PH
but the module is not available.
Merge pull request #866 from peabody124/ah_reset_fix

Transmitter Control: only set AltHoldDesired when exists