media: dvb-frontends: drx39xyj: remove obsolete sign extend macros

DRX_S9TOS16 and DRX_S24TODRXFREQ are simply not used. Furthermore,
sign_extend32() should be used for sign extension. (Also, the comment
describing DRX_S24TODRXFREQ was wrong). So remove these macros.

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