[][mt76-vendor: fix coverity errors]

[Description]
Fix some coverity errors for mt76-vendor.

[Release-log]
N/A

Change-Id: Iccc74680df03762d1997a6684a109227aed2b27b
Reviewed-on: https://gerrit.mediatek.inc/c/openwrt/feeds/mtk_openwrt_feeds/+/5059083
3 files changed