mailbox: mediatek: Add check for possible failure of kzalloc

The patch 623a6143a845("mailbox: mediatek: Add Mediatek CMDQ driver")
introduce the following static checker warning:
  drivers/mailbox/mtk-cmdq-mailbox.c:366 cmdq_mbox_send_data()
  error: potential null dereference 'task'.  (kzalloc returns null)

Fixes: 623a6143a845 ("mailbox: mediatek: Add Mediatek CMDQ driver")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Houlong Wei <houlong.wei@mediatek.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mtk-cmdq-mailbox.c

@@ -363,6 +363,9 @@ static int cmdq_mbox_send_data(struct mbox_chan *chan, void *data)
 	WARN_ON(cmdq->suspended);
 
 	task = kzalloc(sizeof(*task), GFP_ATOMIC);
+	if (!task)
+		return -ENOMEM;
+
 	task->cmdq = cmdq;
 	INIT_LIST_HEAD(&task->list_entry);
 	task->pa_base = pkt->pa_base;