Fixes the following sparse warning:

drivers/net/dsa/mv88e6xxx/chip.c:2866:5: warning:
 symbol 'mv88e6xxx_g1_set_switch_mac' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>