[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>