Begin abstracting out the RX path in preparation for RX EDMA support.

The RX EDMA support requires a modified approach to the RX descriptor
handling.

Specifically:

* There's now two RX queues - high and low priority;
* The RX queues are implemented as FIFOs; they're now an array of pointers
  to buffers;
* .. and the RX buffer and descriptor are in the same "buffer", rather than
  being separate.

So to that end, this commit abstracts out most of the RX related functions
from the bulk of the driver.  Notably, the RX DMA/buffer allocation isn't
updated, primarily because I haven't yet fleshed out what it should look
like.

Whilst I'm here, create a set of matching but mostly unimplemented EDMA
stubs.

Tested:

  * AR9280, station mode

TODO:

  * Thorough AP and other mode testing for non-EDMA chips;
  * Figure out how to allocate RX buffers suitable for RX EDMA, including
    correctly setting the mbuf length to compensate for the RX descriptor
    and completion status area.

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