PCI/DOE: Make mailbox creation API private

The PCI core has just been amended to create a pci_doe_mb struct for
every DOE instance on device enumeration.  CXL (the only in-tree DOE
user so far) has been migrated to use those mailboxes instead of
creating its own.

That leaves pcim_doe_create_mb() and pci_doe_for_each_off() without any
callers, so drop them.

pci_doe_supports_prot() is now only used internally, so declare it
static.

pci_doe_destroy_mb() is no longer used as callback for
devm_add_action(), so refactor it to accept a struct pci_doe_mb pointer
instead of a generic void pointer.

Because pci_doe_create_mb() is only called on device enumeration, i.e.
before driver binding, the workqueue name never contains a driver name.
So replace dev_driver_string() with dev_bus_name() when generating the
workqueue name.

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: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://lore.kernel.org/r/64f614b6584982986c55d2c6229b4ee2b276dd59.1678543498.git.lukas@wunner.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

.clang-format

@@ -520,7 +520,6 @@ ForEachMacros:
   - 'of_property_for_each_string'
   - 'of_property_for_each_u32'
   - 'pci_bus_for_each_resource'
-  - 'pci_doe_for_each_off'
   - 'pcl_for_each_chunk'
   - 'pcl_for_each_segment'
   - 'pcm_for_each_format'