V4L/DVB (4890): Lgdt330x: fix signal / lock status detection bug

In some cases when using VSB, the AGC status register has been known to
falsely report "no signal" when in fact there is a carrier lock.  The
datasheet labels these status flags as QAM only, yet the lgdt330x
module is using these flags for both QAM and VSB.
This patch allows for the carrier recovery lock status register to be
tested, even if the agc signal status register falsely reports no signal.
Thanks to jcrews from #linuxtv in irc, for initially reporting this bug.

Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>