[][kernel][common][eth][Fix compile warning messages]

[Description]
Fix compile warning messages.

If without this patch, the compiler may generate numerous warning
messages during the compilation process.

[Release-log]
N/A


Change-Id: I3316e84f70aa091ccc8a23221e800c0827c1a1ad
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7636005
4 files changed