[media] cx231xx: Set the power mode instead of using the digital mux GPIOs

The Exeter hardware design does not use GPIOs to manage whether its in
digital mode or analog mode, but we need to setup the power control properly.

For that board, setup power control and remove the mux select call.  It is
highly likely that this change could be used by other boards as well, which
would make power management cleaner (fixing known issues transitioning between
analog and digital mode).

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