[media] dvb_frontend: merge duplicate dvb_tuner_ops.release implementations

Most release callback functions are identical: free the "tuner_priv"
and clear it.  Let's eliminate some bloat by providing this simple
implementation in the dvb_frontend library.

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