iwlwifi: make sta lock private & BH lock

Now that the transport has its own locking,
there's no need to have the sta_lock in the
shared data. Also, it can be a BH lock as
it's not used from IRQ handlers.

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>