Correctly initialise the TXQ link pointer to the last descriptor in the last buffer in the list.
Correctly initialise the TXQ link pointer to the last descriptor in
the last buffer in the list.

The current behaviour (due to me, so pointy hat is firmly on my head here)
was incorrect - it was setting the link pointer to the last descriptor
of the _first_ buffer in the TXQ.  Instead, it should have set it to the
last descriptor in the _last_ buffer in the TXQ.

This showed up as occasional TX stalls with frames in the TXQ but no
TX progress being made.  Further inspection showed the TXQ looked like
it contained multiple "lists" of frames - there'd be a list of correct
frames, then a NULL link pointer, but there'd be a next buffer in the
list.

Since this code is only called upon an interface reset, it's likely
this only began showing up when I started doing stress testing
in environments which annoy the radios enough to cause lockups.

I've not yet any TX stalls with this patch applied.

PR:		kern/165866

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase