[][kernel][mt7988][eth][Add USXGMII 5G force mode support]

[Description]
Add USXGMII 5G force mode support.

If without this patch, USXGMII cannot be configured as 5G force mode.

[Release-log]
N/A


Change-Id: I910fba2acede2f1754fb34273141c77d4e870538
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7036929
5 files changed