media: mxl692: MaxLinear 692 ATSC demod/tuner driver

MaxLinear 692 is a combo demod/tuner which has ATSC and QAM
capabilities. Only ATSC is currently advertised via DVB
properties. QAM still has issues.

Confirmed working on both big and little endian. Big
endian verified on PowerPC Mac mini.

Signed-off-by: Brad Love <brad@nextdimension.cc>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>