[][Add proprietary ioctl back to mii_mgr]

[Description]
Add proprietary ioctl back to mii_mgr.

[Release-log]
N/A

Change-Id: Iddf0a13fb11a49fb16620ccdbb2c58a1940d7c69
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/4538724
3 files changed