[media] dvb_frontend: Simplify the emulation logic

The current logic was broken and too complex; while it works
fine for DVB-S2/DVB-S, it is broken for ISDB-T.
Make the logic simpler, fixes it for ISDB-T and make it clearer.

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