iwlwifi: mvm: rs: fallback to legacy Tx columns

Allow switching back to legacy Tx columns so we'll stop doing
HT/VHT in case we're far from the AP. Stop active aggregation when
making a deciding to stay in a legacy column.
Despite having low legacy rates in the LQ table lower entries
it doesn't help much in case we're doing aggregations as the
aggregation was being transmitted in the initial rate of the table.

This should help traffic stalls when far from the AP.

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>