Indicate Barker Preamble Mode in ERP IE also based on local configuration

While this is not strictly speaking required based on dynamic
configuration (i.e., dot11ShortPreambleOptionImplemented is static
value based on implementation, not runtime configuration), it is better
to follow local configuration parameter for short preamble in addition
to the associated station capabilities.

src/ap/beacon.c

@@ -57,7 +57,8 @@ static u8 ieee802_11_erp_info(struct hostapd_data *hapd)
 		}
 		break;
 	}
-	if (hapd->iface->num_sta_no_short_preamble > 0)
+	if (hapd->iface->num_sta_no_short_preamble > 0 ||
+	    hapd->iconf->preamble == LONG_PREAMBLE)
 		erp |= ERP_INFO_BARKER_PREAMBLE_MODE;
 
 	return erp;