AP_Baro: rename macros to avoid conflicts

these macros were also defined in NuttX in clock.h

libraries/AP_Baro/AP_Baro_BMP280.cpp

@@ -115,7 +115,7 @@ bool AP_Baro_BMP280::_init()
     _dev->get_semaphore()->give();
 
     // request 50Hz update
-    _dev->register_periodic_callback(20 * USEC_PER_MSEC, FUNCTOR_BIND_MEMBER(&AP_Baro_BMP280::_timer, void));
+    _dev->register_periodic_callback(20 * AP_USEC_PER_MSEC, FUNCTOR_BIND_MEMBER(&AP_Baro_BMP280::_timer, void));
 
     return true;
 }

libraries/AP_Baro/AP_Baro_MS5611.cpp

@@ -144,7 +144,7 @@ bool AP_Baro_MS56XX::_init()
     _dev->get_semaphore()->give();
 
     /* Request 100Hz update */
-    _dev->register_periodic_callback(10 * USEC_PER_MSEC,
+    _dev->register_periodic_callback(10 * AP_USEC_PER_MSEC,
                                      FUNCTOR_BIND_MEMBER(&AP_Baro_MS56XX::_timer, void));
     return true;
 }