ACPI: PCC: Fix Tx acknowledge in the PCC address space handler

Currently, mbox_client_txdone() is called from the PCC address space
handler and that expects the user the Tx state machine to be controlled
by the client which is not the case and the below warning is thrown:

  | PCCT: Client can't run the TX ticker

Let the controller run the state machine and the end of Tx can be
acknowledge by calling mbox_chan_txdone() instead.

Fixes: 77e2a047 ("ACPI: PCC: Implement OperationRegion handler for the PCC Type 3 subtype")
Signed-off-by: Huisong Li <lihuisong@huawei.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>