b44: abort when no PHY is available at all

When the phy address is 31, this means that there is no PHY connected
to this MAC at all, no internal and no external PHY. Reading these PHY
registers causes a system reset on some routers.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: David S. Miller <davem@davemloft.net>