PCI/DOE: Fix memory leak with CONFIG_DEBUG_OBJECTS=y

After a pci_doe_task completes, its work_struct needs to be destroyed
to avoid a memory leak with CONFIG_DEBUG_OBJECTS=y.

Fixes: 9d24322e ("PCI/DOE: Add DOE mailbox support functions")
Tested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: stable@vger.kernel.org # v6.0+
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/775768b4912531c3b887d405fc51a50e465e1bf9.1678543498.git.lukas@wunner.de


Signed-off-by: Dan Williams <dan.j.williams@intel.com>