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

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

[Release-log]
N/A


Change-Id: I440dc23f1fbc2e1a5cf86afdd1112d77156819aa
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/7398188
1 file changed