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