[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>

drivers/media/dvb-frontends/cx24120.c

@@ -838,7 +838,7 @@ static int cx24120_get_fec(struct dvb_frontend *fe)
 }
 
 /* Calculate ber window time */
-void cx24120_calculate_ber_window(struct cx24120_state *state, u32 rate)
+static void cx24120_calculate_ber_window(struct cx24120_state *state, u32 rate)
 {
 	struct dvb_frontend *fe = &state->frontend;
 	struct dtv_frontend_properties *c = &fe->dtv_property_cache;