media: dvb-frontend/mxl5xx: add support for physical layer scrambling

The MaxLinear MxL5xx has support for physical layer scrambling, which was
recently added to the DVB core via the new scrambling_sequence_index
property. Add required bits to the mxl5xx driver.

Picked up from dddvb master, commit 5c032058b9ba ("add support for PLS")
by Ralph Metzler <rjkm@metzlerbros.de>, adapted to the different naming
of the pls property (pls vs. scrambling_sequence_index).

Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>