Add a write_field() function that acts as helper to update specific bits
specified in the field defines (FSTV0910_*) in stv0910_regs.h, which was
recently updated to carry the missing offset values. With that, add the
SET_FIELD(), SET_REG() and GET_REG() macros that wrap the write_field(),
write_reg() and read_reg() functions to allow for making all demod
access code cleaner.

The write_field() function is annotated with __maybe_unused temporarily
to silence eventual compile warnings.

Picked up from the dddvb upstream, with the macro names made uppercase
so they are distinguishable as such.

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>