iwlwifi: mvm: enable SF also when we have HE

We shouldn't do this just for HT/VHT, but be future-proof
and also check for HE.

Change-Id: Icaeff714c00a773681dbfee72558afd1c7121c5d
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>