[media] it913x-fe ver 1.15 read signal strenght using reg VAR_P_INBAND

Read signal strength using VAR_P_INBAND and apply FEC preferred values.

Note this does not work on IT9137 devices even with dvb-usb-it9135-01.fw
firmware.

Config read_sl allows switch between read signal strength and signal
level.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>