V4L/DVB (4493): [lnbp21] release callback fixed

lnbp21 module must use fe->ops.release_sec, not fe->ops.release.
The release function of the frontend driver was never called for cards
with lnbp21...

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>