media: dvb-frontends/stv0910: deduplicate writes in enable_puncture_rate()

For all code rates, the same write is performed, only with a differing
value. Clean this up by putting that value into a variable instead and
perform the write at the end with that value.

Picked up from the dddvb upstream.

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>