mailbox: mediatek: Add check for possible failure of kzalloc

The patch 623a6143("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: 623a6143

 ("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>