mac80211: make uapsd_* keys per-vif

uapsd_queues and uapsd_max_sp_len are relevant only for managed
interfaces, and can be configured differently for each vif.

Move them from the local struct to sdata->u.mgd, and update
the debugfs functions accordingly.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>