[sbus] Fix SBUS failsafe bit similair to other projects (#2702)

sw/airborne/subsystems/radio_control/sbus.c

@@ -76,7 +76,7 @@ void radio_control_impl_event(void (* _received_frame_handler)(void))
   if (sbus.frame_available) {
     //With Sbus one can still have a receiver sent valid frames generated by the
     //failsafe mode of the receiver, having frames in this case does not guarantee having RC link
-    if (sbus.rc_lost) {
+    if (sbus.rc_lost || sbus.rc_failsafe) {
       radio_control.status = RC_REALLY_LOST;
       if (radio_control.radio_ok_cpt > 0) {
         radio_control.radio_ok_cpt--;

sw/airborne/subsystems/radio_control/sbus_common.c

@@ -44,7 +44,7 @@
 #define SBUS_BIT_PER_BYTE 8
 #define SBUS_FLAGS_BYTE 22
 #define SBUS_FRAME_LOST_BIT 2
-#define SBUS_RC_FAILSAFE_BIT 4
+#define SBUS_RC_FAILSAFE_BIT 3
 #define SBUS_RC_LOST_BIT 5
 
 #define SBUS_STATUS_UNINIT 0