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>