[][kernel][mt7988][eth][Refactor PCS programming sequence for the USXGMII]

[Description]
Refactor PCS programming sequence for the USXGMII.

Merge PCS programming sequence of the USXGMII/10GBASER/5GBASER to the
one.

If without this patch, the USXGMII driver cannot use the same
programming sequence to configure different PCS mode.

[Release-log]
N/A


Change-Id: I2011044c411252015818c14f2f9d621c1f995e2d
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7507247
3 files changed