[media] ivtv: remove exclusive radio open

For some reason the /dev/radio device was implemented as an exclusive open:
you could open it only once and not a second time.

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>