[][mt76-vendor: fix more coverity errors]

[Description]
Fix some more coverity errors related to NULL check for mt76-vendor.

[Release-log]
N/A

Change-Id: I096dd26bb6662d4f3e495707231f9ba482e0cc9a
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5072762
3 files changed