net: bgmac: fix reversed check for MII registration error

It was failing on successful registration returning meaningless errors.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Fixes: 55954f3b ("net: ethernet: bgmac: move BCMA MDIO Phy code into a separate file")
Signed-off-by: David S. Miller <davem@davemloft.net>