Prepare ath5k for WME by using four hardware queues.

The way we set up our queues matches the mac80211 queue priority 1:1, so we
don't have to do any mapping for queue numbers.

Every queue uses 50 of the total 200 available transmit buffers, so the DMA
memory usage does not increase with this patch, but it might be good to
fine-tune the number of buffers per queue later (depending on the CPU speed and
load, and the speed of the medium access, it might not be big enough).
Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>