iwlwifi: update BT traffic load states correctly

When BT traffic load changes from its
previous state, a new LQ command needs to be
sent down to the firmware. This needs to
be done only once per change. The state
variable that keeps track of this change is
last_bt_traffic_load. However, it was not
being updated when the change had been
handled. Not updating this variable was
causing a flood of advanced BT config
commands to be sent to the firmware. Fix
this.

Cc: stable@vger.kernel.org #2.6.38+
Signed-off-by: Meenakshi Venkataraman <meenakshi.venkataraman@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>