[media] em28xx: declare em28xx_stop_streaming as static

That fixes the following warning:
	drivers/media/usb/em28xx/em28xx-video.c:611:5: warning: no previous prototype for 'em28xx_stop_streaming' [-Wmissing-prototypes]
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/em28xx/em28xx-video.c

@@ -608,7 +608,7 @@ int em28xx_start_analog_streaming(struct vb2_queue *vq, unsigned int count)
 	return rc;
 }
 
-int em28xx_stop_streaming(struct vb2_queue *vq)
+static int em28xx_stop_streaming(struct vb2_queue *vq)
 {
 	struct em28xx *dev = vb2_get_drv_priv(vq);
 	struct em28xx_dmaqueue *vidq = &dev->vidq;