dpp: fail early if multicast frame registration is unsupported

This is done already for DPP, do the same for PKEX. Few drivers
(ath9k upstream, ath10k/11k in progress) support this which is
unfortunate but since a configurator will not work without this
capability its best to fail early.

src/dpp.c

@@ -4362,6 +4362,12 @@ static struct l_dbus_message *dpp_start_pkex_configurator(struct dpp_sm *dpp,
 				dpp->interface != DPP_INTERFACE_UNBOUND)
 		return dbus_error_busy(message);
 
+	if (!dpp->mcast_support) {
+		l_debug("Multicast frame registration not supported, cannot "
+			"start a configurator");
+		return dbus_error_not_supported(message);
+	}
+
 	if (!network || !bss)
 		return dbus_error_not_connected(message);