V4L/DVB (13331): mxl5005s: provide ability to override QAM gain for HVR-1600

The HVR-1600 doesn't use the standard mechanism for computing the gain when
in QAM mode, instead always forcing it to be 0x02.  Provide the ability to
override the algorithm on a per-board basis.

This change results in an improvement of 0.4-0.6 dB for QAM tuning.

This work was sponsored by ONELAN Limited.

Cc: Steven Toth <stoth@kernellabs.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>