iwlwifi: mvm: choose an initial tx rate based on rssi conditions

Improve the initial tx rate and antenna selection to be based on
the rssi of the last rx. This avoids starting at the lowest legacy
rate always and requiring more tx traffic to "climb" up the rates.
Since this option might cause trouble in certain setups, allow to
disable it by default.

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>