lan743x: Expand phy search for LAN7431

The LAN7431 uses an external phy, and it can be found anywhere in
the phy address space. This patch uses phy address 1 for LAN7430
only. And searches all addresses otherwise.

Signed-off-by: Bryan Whitehead <Bryan.Whitehead@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>