[media] cx18: fix kernel oops when setting MPEG control before capturing

The cxhdl->priv field was not set initially, only after capturing started.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>