These (and a few others) will differ based on the underlying DMA
implementation.
For the EDMA NICs, simply stub them out in a fashion which will let
me focus on implementing the necessary descriptor API changes.
* Introduce TX DMA setup/teardown methods, mirroring what's done in
the RX path.
Although the TX DMA descriptor is setup via ath_desc_alloc() /
ath_desc_free(), there TX status descriptor ring will be allocated
in this path.
* Remove some of the TX EDMA capability probing from the RX path and
push it into the new TX EDMA path.
