Make sure asynchronous commands do not race with synchronous ones.

Make sure asynchronous commands do not race with synchronous ones.

Since asynchronous commands can be submitted from interrupt context
it was possible to race with a process waiting for the completion of
a previously submitted command.  So stop relying on the per-softc
TRB pointer for asynchronous commands and simply get the address of
the command TRB from the event TRB.