[][mii_mgr: fix coverity errors]

[Description]
Fix CERT ERR33-C coverity errors.
Also fix the following CR:




[Release-log]
N/A

Change-Id: I86e00c7f71f1334d95215fcafdec1964882de29e
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5892517
1 file changed