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

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 <jassisinghbrar@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260322171533.608436-1-jassisinghbrar@gmail.com