V4L/DVB (10759): cx18: Convert GPIO connected functions to act as v4l2_subdevices

Convert GPIO line functions, such a audio routing and device resets, to
v4l2_subdevices.  This essentially completes the conversion of cx18 to the
v4l2_device/v4l2_subdevice framework.  No regression testing has taken place as
of yet.  Also an ivtv legacy bug with GPIO mux routing and going to/from radio
mode was commented, but not fixed.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>