[][kernel][common][eth][Fix issues detected by Coverity scan]

[Description]
Fix issues detected by Coverity scan.

1.mtk_device_event: DEADCODE
2.mtk_mac_finish: MISSING_LOCK
3.mtk_usxgmii_pcs_link_up: MISSING_LOCK

[Release-log]
N/A


Change-Id: Ic96f8bb0ef32ff9935e2a7994ce37db60fcf6120
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/8853534
2 files changed