mpu9250: fix mag publishing garbage on IMU failure (SPI only)

The Pixhawk cube as an internal mag on SPI. If there was a bad register
read (e.g. on an IMU failure), _register_wait was non-zero and there was
no transfer but _mag still published mpu_report.mag.

src/drivers/imu/mpu9250/mpu9250.cpp

@@ -1058,7 +1058,9 @@ MPU9250::measure()
 	if (_mag->is_passthrough()) {
 #   endif
 
-		_mag->_measure(mpu_report.mag);
+		if (_register_wait == 0) {
+			_mag->_measure(mpu_report.mag);
+		}
 
 #   ifdef USE_I2C