irqchip/qcom-mpm: Add missing mailbox TX done acknowledgment

The mbox_client for qcom-mpm sends NULL doorbell messages via
mbox_send_message() but never signals TX completion.

Set knows_txdone=true and call mbox_client_txdone() after a successful
send, matching the pattern used by other Qualcomm mailbox clients (smp2p,
smsm, qcom_aoss etc).

Fixes: a6199bb514d8a6 "irqchip: Add Qualcomm MPM controller driver"
Signed-off-by: Jassi Brar <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Reviewed-by: Douglas Anderson <[e-mail]>
Cc: [e-mail]
Link: https://patch.msgid.link/[e-mail]

drivers/irqchip/irq-qcom-mpm.c

···
 	if (ret < 0)
 		return ret;
 
+	mbox_client_txdone(priv->mbox_chan, 0);
+
 	return 0;
 }
 
···
 	}
 
 	priv->mbox_client.dev = dev;
+	priv->mbox_client.knows_txdone = true;
 	priv->mbox_chan = mbox_request_channel(&priv->mbox_client, 0);
 	if (IS_ERR(priv->mbox_chan)) {
 		ret = PTR_ERR(priv->mbox_chan);