The AR9300 and later descriptors are 128 bytes, however I'd like to make
sure that isn't used for earlier chips.
* Populate the TX descriptor length field in the softc with
sizeof(ath_desc)
* Use this field when allocating the TX descriptors
* Pre-AR93xx TX/RX descriptors will use the ath_desc size; newer ones will
query the HAL for these sizes.