media: uvcvideo: Split uvc_video_enable into two

uvc_video_enable() is used both to start and stop the video stream
object, however the single function entry point shares no code between
the two operations.

Split the function into two distinct calls, and rename to
uvc_video_start_streaming() and uvc_video_stop_streaming() as
appropriate.

Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>