V4L/DVB (5826): Usbvision: video mux cleanup

- usbvision_muxsel simplified, now uses some well known constants.
- since the decoder needs to change input norm, call to muxsel added when
  changing video standard.

Signed-off-by: Thierry MERLE <thierry.merle@free.fr>
Acked-by: Dwaine Garden <DwaineGarden@rogers.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>