Plane: prevent baro reset from OVERRIDE_CHAN

using OVERRIDE_CHAN would cause a baro reset as it would mean we are
temporarily disarmed, and the "reset baro and GPS when disarmed" check
would reset the baro

Now we only do the mixer update if disarmed

ArduPlane/control_modes.pde

@@ -65,7 +65,10 @@ static void read_control_switch()
         // if the user has configured an override channel then check it
         bool override = (hal.rcin->read(g.override_channel-1) >= PX4IO_OVERRIDE_PWM);
         if (override && !px4io_override_enabled) {
-            if (setup_failsafe_mixing()) {
+            // we only update the mixer if we are not armed. This is
+            // important as otherwise we will need to temporarily
+            // disarm to change the mixer
+            if (ahrs.get_armed() || setup_failsafe_mixing()) {
                 px4io_override_enabled = true;
                 // disable output channels to force PX4IO override
                 for (uint8_t i=0; i<16; i++) {