V4L/DVB (7931): cx18: allow for simultaneous digital and analog capture

The HVR-1600 can do both analog and digital capture at the same time.
Due to a driver bug -EBUSY would be returned when attempting to setup an
analog capture while a digital capture was already in progress.

Separate the two internally.

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