[media] cxd2841er: fix switch-case for DVB-C

DVB-C should use cxd2841er_read_agc_gain_c() to get the gain. The same
function is used for all DVB-C annex delivery systems.

Signed-off-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>