ath79: fix link mode support list on UniFi AC

ath79: fix link mode support list on UniFi AC

This fixes incorrect supported link modes indicated when using ethtool
on the Ubiquiti UniFi AC lite / LR / Mesh.

eroot@OpenWrt:~# ethtool eth0
Settings for eth0:
	Supported ports: [ TP MII ]
	Supported link modes:   100baseT/Half 100baseT/Full
	                        1000baseT/Full
	                        1000baseX/Full

The reason for this is, that since kernel 5.4 the PHY capabilities are
read dynamically from the PHY. As Ubiquiti leaves the PHY in a wrong
state, the capabilities of the SGMII side of the PHY are read.

As a side effect, 10 Mbit/s link modes did not work on these boards.

Signed-off-by: David Bauer <mail@david-bauer.net>