The holding buffer logic needs to be used for _all_ transmission, not
just "when the queue is busy."

After talking with the MAC team, it turns out that the linked list
implementation sometimes will not accept a TxDP update and will
instead re-read the link pointer.  So even if the hardware has
finished transmitting a chain and has hit EOL/VEOL, it may still
re-read the link pointer to begin transmitting again.

So, always set ATH_BUF_BUSY on the last buffer in the chain (to
mark the last descriptor as the holding descriptor) and never
blank the axq_link pointer.

Tested:

* AR5416, STA mode

TODO:

* much more thorough testing with the pre-11n NICs, just to verify
  that they behave the same way.
* test TDMA on the 11n and non-11n hardware.