mailbox: pcc: Mark Tx as complete in PCC IRQ handler

The PCC IRQ handler clears channel-in-use and notifies clients with
mbox_chan_received_data(), but it does not explicitly mark the
transmit as complete. In IRQ completion mode this could leave Tx complete
waiters hanging or lead to generic timeouts in the mailbox core.

Invoke mbox_chan_txdone() in the IRQ path once the platform has
acknowledged the transfer so the core can wake any waiters and update
state accordingly.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Adam Young <admiyo@os.amperecomputing.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>