iwlwifi: mvm: add an option to start rs from HT/VHT rates

Extend the configurable option of setting initial rate to RSSI based.
Make the initial rate to be set to VHT/HT SISO or legacy depending on
the AP capabilities.

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>