arge_mdio: fix barriers; correctly check MII indicator register.

arge_mdio: fix barriers; correctly check MII indicator register.

* use barriers in a slightly better fashion.  You can blame this
  glass of whiskey on putting barriers in the wrong spot.  Grr adrian.

* steal/rewrite the mdio busy check from ag7100 from openwrt and
  refactor the existing code out.  This is .. more correct.

This seems to fix the boot-to-boot variation that I've been seeing
and it quietens the switch port status flapping.

Tested:

* QCA9558 SoC (AP135.)

Obtained from:	Linux OpenWRT