mailbox: sprd: clear delivery flag before handling TX done

mailbox: sprd: clear delivery flag before handling TX done

If there are any pending messages in the mailbox queue, they are sent
as soon as a TX done event arrives from the driver. This may trigger a
new delivery interrupt while the previous one is still being handled.
If the delivery status is cleared after this, the interrupt is lost.
To prevent this from happening, clear the delivery status immediately
after checking it and before any new messages are sent.

Signed-off-by: Otto Pflüger <otto.pflueger@abscue.de>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>