asix: Add a new driver for the AX88172A

The Asix AX88172A is a USB 2.0 Ethernet interface that supports both an
internal PHY as well as an external PHY (connected via MII).

This patch adds a driver for the AX88172A and provides support for
both modes and the phylib.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: David S. Miller <davem@davemloft.net>