ax88796: start_xmit fix using net_device_ops

This patch hooks up the start_xmit/tx_timeout/get_stats callbacks
in the ax88796 driver since they no longer are installed by the
lib8390 code. Without this patch the function dev_hard_start_xmit()
crashes due to a start_xmit callback with the value NULL.

While at it, update the ax88796 driver to make use of use of struct
net_device_ops.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>