[][mii_mgr: fix coverity errors]

[Description]
Fix BUFFER_SIZE error reported by coverity.

[Release-log]
N/A

Change-Id: I06d34e2fbc90e1e59171bba8cc77916305826213
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5395187
1 file changed