V4L/DVB (9805): cx18: Port fix for raw/sliced VBI mixup from ivtv and cx25840

This is a port of the fixes Hans Verkuil made for ivtv/cx25840:
The service_set field was used to determine whether raw or sliced VBI was
desired. This is incorrect since it is perfectly valid to select sliced VBI
with a service_set of 0.

Instead the driver should check on VIDIOC_S_FMT whether the type
field matches the raw or sliced VBI type.

Updated the cx18 driver accordingly, including an additional check in
cx18_start_v4l2_encode_stream() that didn't exist in ivtv.

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