V4L/DVB (11309): cx25840: cleanup: remove intermediate 'ioctl' step

The audio and vbi functions where still called through an ioctl-like
interface, even though this is no longer needed with v4l2-subdev. Just
change each 'case' into a proper function and call that directly.

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