net: dsa: bcm_sf2: Fix non static symbol warning

Fixes the following sparse warning:

drivers/net/dsa/bcm_sf2.c:963:19: warning:
 symbol 'bcm_sf2_io_ops' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/bcm_sf2.c

@@ -960,7 +960,7 @@ static int bcm_sf2_core_write64(struct b53_device *dev, u8 page, u8 reg,
 	return 0;
 }
 
-struct b53_io_ops bcm_sf2_io_ops = {
+static struct b53_io_ops bcm_sf2_io_ops = {
 	.read8	= bcm_sf2_core_read8,
 	.read16	= bcm_sf2_core_read16,
 	.read32	= bcm_sf2_core_read32,