ath9k: Fix sequence number assignment

Currently, ath9k uses a global counter for all
frames that need to be assigned a sequence number.
QoS-data frames are handled properly since they
have a per-tid counter. But, beacons and other
management frames use the same counter even if
multiple interfaces or contexts are present.

Fix this issue by making the counter per-interface
and using it when mac80211 sets IEEE80211_TX_CTL_ASSIGN_SEQ.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>