net: phy: improve get_phy_id

Only caller of get_phy_id() is get_phy_device(). There a PHY ID of
0xffffffff is translated back to -ENODEV. So we can avoid some
overhead by returning -ENODEV directly.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>