mpu9250:mag rework the setup to veify HW first

If the setup is unsucessful fo not register the devices or
   allocate resources.

src/drivers/mpu9250/mag.cpp

@@ -123,7 +123,9 @@ MPU9250_mag::~MPU9250_mag()
 int
 MPU9250_mag::init()
 {
-	int ret;
+	int ret = ak8963_setup();
+
+	if (ret == OK) {
 
 		ret = CDev::init();
 
@@ -141,7 +143,6 @@ MPU9250_mag::init()
 
 		_mag_class_instance = register_class_devname(MAG_BASE_DEVICE_PATH);
 
-	ak8963_setup();
 
 		/* advertise sensor topic, measure manually to initialize valid report */
 		struct mag_report mrp;
@@ -154,6 +155,7 @@ MPU9250_mag::init()
 		if (_mag_topic == nullptr) {
 			warnx("ADVERT FAIL");
 		}
+	}
 
 out:
 	return ret;