dvbscan: seek for both DVB-T and DVB-T2 on DVB-T2 autoscan

At least on some devices, setting the device to DVB-T2 and
seeking for a DVB-T channel won't find anything. So, we'll
need to explicitly seek for both DVB-T and DVB-T2.

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