[media] dvb_frontend: tuner_ops.release returns void

It is not clear what this return value means.  All implemenations
return 0, and the one caller ignores the value.  Let's remove this
useless return value completely.

Signed-off-by: Max Kellermann <max.kellermann@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>