mac80211: add queue debugfs file

I suspect that some driver bugs can cause queues to be
stopped while they shouldn't be, but it's hard to find
out whether that is the case or not without having any
visible information about the queues. This adds a file
to debugfs that allows us to see the queues' statuses.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>