net: axienet: fix MDIO bus naming

The MDIO bus for this driver was being named using the result of
of_address_to_resource on a node which may not have any resource on it,
but the return value of that call was not checked so it was using some
random value in the bus name. Change to name the MDIO bus based on the
resource start of the actual Ethernet register block.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>