Break out the TX descriptor link field into HAL methods.

Break out the TX descriptor link field into HAL methods.

The DMA FIFO chips (AR93xx and later) differ slightly to th elegacy
chips:

* The RX DMA descriptors don't have a ds_link field;
* The TX DMA descriptors have a ds_link field however at a different
  offset.

This is a reimplementation based on what the reference driver and ath9k
does.

A subsequent commit will enable it in the TX and beacon paths.

Obtained from:	Linux ath9k, Qualcomm Atheros