media: dvb-frontends/stv0910: cleanup init_search_param() and enable PLS

Cleanup the mess in init_search_param() by utilising the new register
access macros and functions. And while at it, move the ISI and PLS setup
into separate functions, and pass the new scrambling_sequence_index (aka.
physical layer scrambling) value to set_pls.

Picked up from the dddvb upstream, 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>