mt7925: fix NULL pointer dereference in vif iteration loops

[zbowling]

I was having a kernel panic in the wifi driver on my new Framework Desktop after 15-20 minutes of using it. I sent this patch upstream to try and resolve the nullptr deref. Figured it makes sense to include here too. I also filed a bug with Ubuntu with some details.

https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2137291

2-0001-wifi-mt76-mt7925-fix-NULL-pointer-dereference-in-vif.patch

https://lore.kernel.org/all/CAA5_Hq7vNOy9oCGkkgyukq2OP=a5yL_3ZKBdmNtBXS+zp6byiQ@mail.gmail.com/T/#u

[zbowling]

It took another patch to fix this, too, because of a discovered deadlock after fixing the panic. Maintaining this repo of the patches:

https://github.com/zbowling/mt7925