Pull up following revision(s) (requested by riastradh in ticket #1942):

sys/dev/usb/if_urtwn.c: revision 1.109 (patch)

urtwn(4): Ditch old queued commands on overflow.
Don't increment ring->queued past what the task will decrement.

This is a stop-gap measure; really, we should just have one task for
each operation that is deferred to the task thread.

PR kern/57965

(martin)