net: asix: ax88772: migrate to phylink

There are some exotic ax88772 based devices which may require
functionality provide by the phylink framework. For example:
- US100A20SFP, USB 2.0 auf LWL Converter with SFP Cage
- AX88772B USB to 100Base-TX Ethernet (with RMII) demo board, where it
  is possible to switch between internal PHY and external RMII based
  connection.

So, convert this driver to phylink as soon as possible.

Tested with:
- AX88772A + internal PHY
- AX88772B + external DP83TD510E T1L PHY

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>