V4L/DVB (7579): bttv: Fix memory leak in radio_release

Fix the leak of the bttv_fh structure allocated in radio_open which
was introduced by commit 5cd3955c.

Signed-off-by: Robert Fitzsimons <robfitz@273k.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>