iwlagn: let bluetooth traffic load impact rate scale

Depending on the amount of bluetooth traffic,
using the shared antenna (antenna B) will have
adverse impact on both bluetooth and wireless
traffic. Add controls to improve the situation
by making rate scaling depend on the BT load.

When there's high bluetooth traffic load, there's
little point in trying to aggregate as BT traffic
would disrupt the aggregated frames all the time,
so simply don't start sessions then.

When BT traffic returns to lower levels, the rate
scaling will come here again automatically when
wifi traffic is high enough, and then it will be
able to successfully enable aggregation.

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>