[media] stb0899: fixed reading of IF_AGC_GAIN register

When reading IF_AGC_GAIN register a wrong value for the base address
register was used (STB0899_DEMOD instead of STB0899_S2DEMOD). That
lead to a wrong signal strength value on DVB-S2 transponders.

Signed-off-by: Andreas Regel <andreas.regel@gmx.de>
Acked-by: Klaus Schmidinger <Klaus.Schmidinger@tvdr.de>
Cc: Manu Abraham <abraham.manu@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>