staging/airspy: fix a compilation warning

drivers/staging/media/airspy/airspy.c: In function 'airspy_stop_streaming':
drivers/staging/media/airspy/airspy.c:569:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^

Cc: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/staging/media/airspy/airspy.c

@@ -566,14 +566,13 @@ err:
 static void airspy_stop_streaming(struct vb2_queue *vq)
 {
 	struct airspy *s = vb2_get_drv_priv(vq);
-	int ret;
 
 	dev_dbg(&s->udev->dev, "%s:\n", __func__);
 
 	mutex_lock(&s->v4l2_lock);
 
 	/* stop hardware streaming */
-	ret = airspy_ctrl_msg(s, CMD_RECEIVER_MODE, 0, 0, NULL, 0);
+	airspy_ctrl_msg(s, CMD_RECEIVER_MODE, 0, 0, NULL, 0);
 
 	airspy_kill_urbs(s);
 	airspy_free_urbs(s);