net: dsa: mv88e6xxx: fix non static symbol warnings

Fixes the following sparse warnings:

drivers/net/dsa/mv88e6xxx/chip.c:219:5: warning:
 symbol 'mv88e6xxx_port_read' was not declared. Should it be static?
drivers/net/dsa/mv88e6xxx/chip.c:227:5: warning:
 symbol 'mv88e6xxx_port_write' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>