ath79: force SGMII SerDes mode to MAC operation

ath79: force SGMII SerDes mode to MAC operation

The mode on the SGMII SerDes on the QCA9563 is 1000 Base-X by default.
This only allows for 1000 Mbit/s links, however when used with an SGMII
PHY in 100 Mbit/s link mode, the link remains dead.

This strictly has nothing to do with the SerDes calibration, however it
is done at the same point in the QCA reference U-Boot which is the
blueprint for everything happening here. As the current state is more or
less a hack, this should be fine.

This fixes the issues outlined above on a TP-Link EAP-225 Outdoor.

Reported-by: Tom Herbers <freifunk@tomherbers.de>
Tested-by: Tom Herbers <freifunk@tomherbers.de>
Signed-off-by: David Bauer <mail@david-bauer.net>