V4L/DVB (4532): Isl6421: Release callback installed incorrectly

The release callback was installed in ops.release, it should have been
ops.release_sec.  This prevented to front-end release from getting
called.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>