net: phy: export fixed_phy_register()

When building the bcmgenet driver as module, I get:

ERROR: "fixed_phy_register" [drivers/net/ethernet/broadcom/genet/genet.ko] undefined!

commit b0ba512e225d72 ("net: bcmgenet: enable driver to work without device
tree") which added a call to fixed_phy_register. But fixed_phy_register
needs to be exported if used from a module.

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>