iwlwifi: introduce per-queue locks

Instead of (ab)using the sta_lock, make the
transport layer lock its own TX queue data
structures with a lock per queue. This also
unifies with the cmd queue lock.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>