iwlwifi: mvm: rs: report last tx rate based on RSSI and caps

In scenarios where we haven't converged yet to a specific modulation
and rate it could be better to report to userspace the last tx rate
based on the STA capabilities and RSSI. This is important as sometimes
userspace displays the last tx rate as the link speed.
This avoids being presented with low legacy rates when rs just begins
its search or after an idle period in which it resets itself.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>