PCI/DOE: Deduplicate mailbox flushing

When a DOE mailbox is torn down, its workqueue is flushed once in
pci_doe_flush_mb() through a call to flush_workqueue() and subsequently
flushed once more in pci_doe_destroy_workqueue() through a call to
destroy_workqueue().

Deduplicate by dropping flush_workqueue() from pci_doe_flush_mb().

Rename pci_doe_flush_mb() to pci_doe_cancel_tasks() to more aptly
describe what it now does.

Tested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Ming Li <ming4.li@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/1f009f60b326d1c6d776641d4b20aff27de0c234.1678543498.git.lukas@wunner.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>