net: au1000_eth: fix PHY detection

Commit 7f854420


("phy: Add API for {un}registering an mdio device to a bus.")
broke PHY detection on this driver with a copy-paste bug:
The code is looking 32 times for a PHY at address 0.

Fixes ethernet on AMD DB1100/DB1500/DB1550 boards which have
their (autodetected) PHYs at address 31.

Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>