V4L/DVB (4600): Fix DVB Front-End Signal Strength Inconsistency

The cx22702 returns an 8 bit unshifted value for signal strength; this is 
inconsistent with most other frontends

Signed-off-by: Bradley Derek Kite <bradley.kite@gmail.com>
Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>