net: axienet: Use standard IO accessors

This driver was using in_be32 and out_be32 IO accessors which do not
exist on most platforms. Also, the use of big-endian accessors does not
seem correct as this hardware is accessed over an AXI bus which, to the
extent it has an endian-ness, is little-endian. Switch to standard
ioread32/iowrite32 accessors.

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