[media] dvb-core: fix a regression with MythTV

As reported by Lawrence[1], MythTV 0.24.1 does the wrong thing
with a DVBv5 call: it fills the delivery system with
SYS_UNDEFINED, expecting that the DVB core would work with that.

This used to work by accident, as the DVB core were missing the
check for the supported delivery systems. Yet, fixing it
is easy, so let's add a logic to handle this case, to
provide backward compatibility.

[1] http://patchwork.linuxtv.org/patch/8314/

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>