AP_Compass: make all semaphores recursive

the cost is very similar and this prevents an easy coding error which
can occur on less used code paths

libraries/AP_Compass/AP_Compass_Backend.h

@@ -112,7 +112,7 @@ protected:
     Compass &_compass;
 
     // semaphore for access to shared frontend data
-    HAL_Semaphore_Recursive _sem;
+    HAL_Semaphore _sem;
 
     // Check that the compass field is valid by using a mean filter on the vector length
     bool field_ok(const Vector3f &field);