Removed redundant checks to GPS_ok before setting flight mode to RTL
(this check is already performed inside the set_mode function)
Removed reset of home distance and bearing when GPS lock is lost, it now
remains at the last known value
Previously if set_mode failed it would return the copter to stabilize
mode. With this commit set_mode returns a true/false indicating whether
it succeeded or not so the caller can make the decision as to the
appropriate response which could be to stay in the current flight mode
or try another flight mode.
Randy Mackay
Olivier-ADLER