[media] cx231xx: do not call video_mux as part of isoc setup

You cannot call the video_mux routine from within the isoc setup, because that
code is shared with the digital isoc handler.  This was causing the GPIOs and
power control to be put into the wrong state when starting up digital mode.

Signed-off-by: Devin Heitmueller <dheitmueller@hauppauge.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>