[media] dvb-frontends/cxd2841er: configurable IFAGCNEG

Adds a flag to enable or disable the IFAGCNEG bit in cxd2841er_init_tc().

Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Acked-by: Abylay Ospan <aospan@netup.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>