[media] cx18: convert to unlocked_ioctl

Also added locking around snd_cx18_pcm_ioctl as a precaution
as requested by Andy Walls.

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