[][kernel][common][eth][Fix Coverity scan warning]

[Description]
Fix Coverity scan warning for the uninitialized variable in the
mtk_check_gmac23_idle().

[Release-log]
N/A


Change-Id: I1112546c2803a163cc7f50d272d89631c00fa496
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7487413
1 file changed