net: ethernet: xilinx: Convert xilinx_axienet to module_platform_driver

Follow commit db62f684

. Convert the driver to use the
module_platform_driver() macro which makes the code a bit smaller and
simpler.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>