realtek: pcs: rtl93xx: extend USXGMII config

realtek: pcs: rtl93xx: extend USXGMII config

Our USXGMII config only covers one single register of which kind there
are actually more. We only set a value for 'CFG_QHSG_TXCFG_MAC_CH0' but
there are additional registers for CH1-CH3. Those refer to the 4
USXGMII 'channels'. While the RTL930x part of the SDK doesn't set them
explicitly, from RTK setup SerDes dumps we can see they are usually
similarly set.

The RTL931x part of the SDK actually writes those register explicitly
during USXGMII. We just haven't implemented that so far. Thus, add this
to the USXGMII config for both RTL93xx.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22939
Signed-off-by: Robert Marko <robimarko@gmail.com>