V4L/DVB (11300): cx88: convert to v4l2_subdev.

Convert cx88 to use v4l2_subdev since the old i2c autoprobing mechanism
will be removed.

Added code to explicitly load tvaudio where needed. Also fix the rtc-isl1208
support: since that driver no longer supports autoprobing it has to be
loaded using the new i2c API.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>