lan743x: Enable driver to work with LAN7431

This driver was designed to work with both LAN7430 and LAN7431.
The only difference between the two is the LAN7431 has support
for external phy.

This change adds LAN7431 to the list of recognized devices
supported by this driver.

Updates for v2:
    changed 'fixes' tag to match defined format

fixes: 23f0703c ("lan743x: Add main source files for new lan743x driver")
Signed-off-by: Bryan Whitehead <Bryan.Whitehead@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>