[panic] Race in IEEE802.11 layer towards device drivers

[Hans Petter Selasky]

Hi AK,

I've committed your patches to USB P4. I've made some additional patches.

Can you check and verify everything?

http://p4web.freebsd.org/@@181189?ac=10

Also please compile a kernel with WITNESS enabled to catch any LOR's, hence we 
introduced another mutex.

--HPS