iwlwifi: mvm: don't filter out the first beacon

Enabling beacon filtering should be done only after a beacon
has been received. Doing that too early will cause
disconnections.
This has already been fixed, but the fix didn't take care
about the case where the beacon is received after the
association, it waited only for association which is not
enough.

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