iwlwifi: mvm: new api to get signal strength

A new API that replaces the rx signal strength calculation via agc & rssi.
The energy is now calculated outside the driver and transferred by the fw.

Signed-off-by: Avri Altman <avri.altman@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>