iwlwifi: mvm: support beacon statistics for BSS client

Report the average beacon signal and the number of received beacons as
measured by the firmware.

Since the firmware just counts, and doesn't reset the counter at all,
clear it in the firmware whenever we associate. However, accumulate it
over firmware restart.

Since clearing the statistics in the firmware will also clear the ones
for the radio statistics, add those to the accumulator when cleared.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>