realtek: dsa: rtl931x: configure phy ability source

realtek: dsa: rtl931x: configure phy ability source

The MAC can get PHY abilities, link status, etc. via different ways. In
RTL931x, the corresponding register needs to be setup properly. By
default, all ports use out-of-band MDIO polling to retrieve that
information. Thus, PHY-backed ports usually work with the default
setting.

For SFP ports, there is no MDIO polling available. Instead, the SerDes
ability bus needs to be used to retrieve the link information.

So far, the bootloader (e.g. U-boot) had to properly initialize that
setting. Instead of relying on that, do that properly during MAC setup.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21351
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>