firmware: arm_scmi: Skip scmi mbox channel setup for addtional devices

Now that the scmi bus supports adding multiple devices per protocol,
and since scmi_create_protocol_device calls scmi_mbox_chan_setup,
we must avoid allocating and initialising the mbox channel if it is
already initialised.
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/driver.c

@@ -735,6 +735,11 @@ static int scmi_mbox_chan_setup(struct scmi_info *info, struct device *dev,
 	idx = tx ? 0 : 1;
 	idr = tx ? &info->tx_idr : &info->rx_idr;
 
+	/* check if already allocated, used for multiple device per protocol */
+	cinfo = idr_find(idr, prot_id);
+	if (cinfo)
+		return 0;
+
 	if (scmi_mailbox_check(np, idx)) {
 		cinfo = idr_find(idr, SCMI_PROTOCOL_BASE);
 		if (unlikely(!cinfo)) /* Possible only if platform has no Rx */