[media] stb0899: use sign_extend32() for sign extension

Instead of implement its own logic, use the already-defined one.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

diff --cc drivers/media/dvb-frontends/stb0899_algo.c
Simple merge