[media] cx18: remove exclusive open of radio device

For some reason the cx18 driver could open the radio device only once.
Remove this limitation.

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