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