V4L/DVB (10502): saa7146: move v4l2 device registration to saa7146_vv.

Doing the v4l2_device registration in the saa7146 core will make it
dependent on v4l2, even for DVB-only boards. This registration and
unregistration belongs in saa7146_vv instead.

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