V4L/DVB: lgdt3305: enable FE_HAS_SIGNAL hack for the lgdt3304 in QAM mode

The signal bit is unreliable on the DT3304 in QAM mode, so
set FE_HAS_SIGNAL based on 'cr_lock'

Signed-off-by: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>