iwlwifi: mvm: don't send the beacon filtering command from iterator

The firmware doesn't allow per-vif beacon filtering: we can
use beacon filtering for one vif only. So remember which
vif has beacon filtering enabled in the iterator, and send
the command outside the iterator.

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