[][kernel][common][eth][Add dsa_port_from_netdev API to dsa framework]

[Description]
Add dsa_port_from_netdev API to dsa framework.

This feature is a backport from kernel-5.15 that can assist the
networking driver to get DSA port from netdev pointer.

[Release-log]
N/A


Change-Id: Ic7af8c766224ed7682bde15cdafb26b674b49767
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8345749
2 files changed