net-next: stmmac: export stmmac_set_mac_addr/stmmac_get_mac_addr

Thoses symbol will be needed for the dwmac-sun8i ethernet driver.
For letting it to be build as module, they need to be exported.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>