[media] dvb-frontend: split set_delivery_system()

This function is complex, and has different workflows, one for
DVBv3 calls, and another one for DVBv5 calls. Break it into 3
functions, in order to make easier to understand what each
block does.
No functional changes so far. A few comments got improved.

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