ath5k: Use capabilities information for the number of TX queues

One thing I missed in my WME series: Older hardware does not have enough
hardware queues to support WME. In this case we just set up one data queue. Use
the capability information to decide how many queues to set up.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>