ath9k: Keep track of stations for debugfs.

The stations hold the ath_node, which holds the tid
and other xmit logic structures.  In order to debug
stuck xmit logic, we need a way to print out the tid
state for the stations.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>