APM: only copy manual channels in failsafe

this prevents non-manual channels changing value away from the trim
value on startup

ArduPlane/failsafe.pde

@@ -39,8 +39,9 @@ void failsafe_check(uint32_t tnow)
         // pass RC inputs to outputs every 20ms
         last_timestamp = tnow;
         APM_RC.clearOverride();
-        for (uint8_t ch=0; ch<8; ch++) {
+        for (uint8_t ch=0; ch<4; ch++) {
             APM_RC.OutputCh(ch, APM_RC.InputCh(ch));
         }
+        RC_Channel_aux::copy_radio_in_out(RC_Channel_aux::k_manual);
     }
 }