iwlwifi: mvm: rs: improve rates table algo

The new logic will attempt more rates with less retries
per rate. Also when starting off with MIMO it will
fallback to SISO with the same MCS and only then to Legacy.
Previously we fell back directly to Legacy.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>