According to documentation higher DCUs have higher priority and should
be used for beacons and CAB traffic. More specifically DCU 9 should be
used for beacons and DCU 8 for CAB traffic, I assumed DCU 7 should be
OK for UAPSD traffic.

Note that DCU 8 and 9 are special because they can only be mapped to a single
QCU each but since we use a 1:1 mapping between QCUs and DCUs anyway we don't
have to change much.

P.S. I also did a few related cleanups on qcu.c and ath5k.h

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>