[media] dvb_frontend: merge the two dvb_frontend_detach() versions

This code duplication is confusing and error prone.  Let's merge them
by moving the release/dvb_detach call into one function with one

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