V4L/DVB (10173): Missing v4l2_prio_close in radio_release

The radio_release function of the BTTV driver is missing a call to
v4l2_prio_close. As a result, after the radio device has been opened at
least once (e.g., by HAL during bootup), v4l2_priority will never drop below
V4L2_PRIORITY_INTERACTIVE again. With the following patch against 2.6.28,
applications that run with V4L2_PRIORITY_BACKGROUND are able to open devices
again. Previous Linux versions are affected as well.

Signed-off-by: Udo Steinberg <udo@hypervisor.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>