PHY capabilities are currently read from the fiber status page, thus
Linux won't advertise 10 / 100 Base-T operation modes, effectively
limiting operation to 1000 Base-T.

Statically set the PHYs capabilities, avoiding autodetection.

The issue itself is properly fixed kernel upstream, however backporting
efforts to OpenWrt master resulted in breaking the fiber operation for
another target.

This is currently only known to be necessary for the Ubiquiti
UniFi AC series, so enabling it in the ath79 target should not
break somewhere else.

--- a/drivers/net/phy/at803x.c
+++ b/drivers/net/phy/at803x.c
@@ -497,7 +497,7 @@ static struct phy_driver at803x_driver[]
 	.get_wol		= at803x_get_wol,
 	.suspend		= at803x_suspend,
 	.resume			= at803x_resume,
-	/* PHY_GBIT_FEATURES */
+	.features		= PHY_GBIT_FEATURES,
 	.read_status		= at803x_read_status,
 	.aneg_done		= at803x_aneg_done,
 	.ack_interrupt		= &at803x_ack_interrupt,