V4L/DVB (10758): cx18: Convert I2C devices to v4l2_subdevices

This is a major perturbation to cx18 I2C device handling to convert it to the
v4l2_device/subdeivce framework.  This change breaks GPIO audio multiplexer
control for the time being.  It will be fixed in a coming change.

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