iwlwifi: mvm: rs: clear per rate stats when aggregation changes

The per rate stats should be cleared when aggregation state changes
to avoid making rate scale decisions based on throughput figures which
were collected prior to the aggregation state change and are now stale.
While at it make sure any clearing of the per rate stats will get logged.

Cc: <stable@vger.kernel.org> [3.14]
Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>