media: lgdt3306a: Add QAM AUTO support

As configured currently, modulation in the driver is set to auto detect,
no matter what the user sets modulation to. This leads to both QAM64
and QAM256 having the same effect. QAM AUTO is explicitly added here for
compatibility with scanning software who can use AUTO instead of doing
essentially the same scan twice.
Also included is a module option to enforce a specific QAM modulation if
desired. The true modulation is read before calculating the snr.
Changes are backwards compatible with current behaviour.

Signed-off-by: Brad Love <brad@nextdimension.cc>
Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>