V4L/DVB (11310): cx18: remove intermediate 'ioctl' step

The audio and vbi parts still used an 'ioctl'-like interface. Replace this
with normal functions.

Cc: Andy Walls <awalls@radix.net>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>