[media] cx24120: fix sparse warning

drivers/media/dvb-frontends/cx24120.c:837:6: warning: symbol 'cx24120_calculate_ber_window' was not declared. Should it be static?

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>