iwlwifi: mvm: support sta_statistics() even on older firmware

The oldest firmware supported by iwlmvm do support getting
the average beacon RSSI. Enable the sta_statistics() call
from mac80211 even on older firmware versions.

Fixes: 33cef925 ("iwlwifi: mvm: support beacon statistics for BSS client")
Cc: stable@vger.kernel.org # 4.2+
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>