Fix Intel X550 IOT issue.
Due to X550 doesn't advertise 2.5G/5G link capability, phlylink
cannot resolve link speed/duplex in the phy_resolve_aneg_linkmode().
Therefore, we handle the link speed from vendor registers.
If without this patch, phylink will show unknown speed messages
when link with NBASE-T link partner.
[Release-log]
N/A
Change-Id: Iaef49fe8ea6ef7c6dc022e50456903b7feb4eef6
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/6852800
cec6012a69