[media] cxd2841er: declare static functions as such

drivers/media/dvb-frontends/cxd2841er.c:992:5: warning: no previous prototype for 'cxd2841er_get_carrier_offset_t2' [-Wmissing-prototypes]
 int cxd2841er_get_carrier_offset_t2(
     ^
drivers/media/dvb-frontends/cxd2841er.c:1032:5: warning: no previous prototype for 'cxd2841er_get_carrier_offset_c' [-Wmissing-prototypes]
 int cxd2841er_get_carrier_offset_c(
     ^
drivers/media/dvb-frontends/cxd2841er.c:1360:5: warning: no previous prototype for 'cxd2841er_read_snr_t2' [-Wmissing-prototypes]
 int cxd2841er_read_snr_t2(struct cxd2841er_priv *priv, u32 *snr)
     ^

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>