Add safety check to nav_rth_home_altitude

[avsaase]

Current Behavior

When the user erroneously sets nav_rth_home_altitude above nav_rth_altitude, the aircraft will climb when it reaches the home position.

Desired Behavior

I think there is no real-world application for having the aircraft climb when it reaches the home position, so this should not be allowed.

Suggested Solution

When nav_rth_home_altitude > nav_rth_altitude, ignore nav_rth_home_altitude. A check in the configurator would also make sense.