Modify the RX path to keep the previous RX descriptor around once it's
used.

It turns out that the RX DMA engine does the same last-descriptor-link-
pointer-re-reading trick that the TX DMA engine.  That is, the hardware
re-reads the link pointer before it moves onto the next descriptor.
Thus we can't free a descriptor before we move on; it's possible the
hardware will need to re-read the link pointer before we overwrite
it with a new one.

Tested:

* AR5416, STA mode

TODO:

* more thorough AP and STA mode testing!
* test on other pre-AR9380 NICs, just to be sure.
* Break out the RX descriptor grabbing bits from the RX completion
  bits, like what is done in the RX EDMA code, so ..
* .. the RX lock can be held during ath_rx_proc(), but not across
  packet input.