phy: fix phy address bug

PHYID returns 0xffff and not 0xffffffff when not found and in some
case(at91sam9263) 0x0. Maybe this patch could be useful.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: David S. Miller <davem@davemloft.net>