Extend the non-aggregate TX descriptor chain routine to be aware of:

Extend the non-aggregate TX descriptor chain routine to be aware of:

* the descriptor ID, and
* the multi-buffer support that the EDMA chips support.

This is required for successful MAC transmission of multi-descriptor
frames.  The MAC simply hangs if there are NULL buffers + 0 length pointers,
but the descriptor did have TxMore set.

This won't be done for the 11n aggregate path, as that will be modified
to use the newer API (ie, ath_hal_filltxdesc() and then set first|middle|
last_aggr), which will deprecate some of the current code.

TODO:

* Populate the numTxMaps field in the HAL, then make sure that's fetched
  by the driver.  Then I can undo that hack.

Tested:

* AR9380, AP mode, TX'ing non-aggregate 802.11n frames;
* AR9280, STA/AP mode, doing aggregate and non-aggregate traffic.