PCI: Fix kernel-doc formatting

Fix kernel-doc formatting throughout drivers/pci and related include files.
No change to functionality intended.

Check for warnings:

$ find include drivers/pci -type f -path "*pci*.[ch]" | xargs scripts/kernel-doc -none

[bhelgaas: squashed to one commit]
Link: https://lore.kernel.org/r/20210509030237.368540-1-kw@linux.com
Link: https://lore.kernel.org/r/20210703151306.1922450-1-kw@linux.com
Link: https://lore.kernel.org/r/20210703151306.1922450-2-kw@linux.com
Link: https://lore.kernel.org/r/20210703151306.1922450-3-kw@linux.com
Link: https://lore.kernel.org/r/20210703151306.1922450-4-kw@linux.com
Link: https://lore.kernel.org/r/20210703151306.1922450-5-kw@linux.com
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

PCI: endpoint: Use "NULL" instead of "0" as a NULL pointer

When returning a NULL pointer, use "NULL" instead of "0".

Fixes sparse warning given by executing "make C=2 drivers/pci/":

CHECK drivers/pci/endpoint/pci-epc-core.c
drivers/pci/endpoint/pci-epc-core.c: note: in included file:
./include/linux/pci-ep-cfs.h:22:16: warning:
Using plain integer as NULL pointer
CHECK drivers/pci/endpoint/pci-epf-core.c
drivers/pci/endpoint/pci-epf-core.c: note: in included file:
./include/linux/pci-ep-cfs.h:31:16: warning:
Using plain integer as NULL pointer

Link: https://lore.kernel.org/r/80895f7465719edb3aa259e907acc4bc3217945c.1600378209.git.gustavo.pimentel@synopsys.com
Reported-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Joao Pinto <jpinto@synopsys.com>

PCI: Add SPDX GPL-2.0 to replace GPL v2 boilerplate

Add SPDX GPL-2.0 to all PCI files that specified the GPL version 2 license.

Remove the boilerplate GPL version 2 language, relying on the assertion in
b24413180f56 ("License cleanup: add SPDX GPL-2.0 license identifier to
files with no license") that the SPDX identifier may be used instead of the
full boilerplate text.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

PCI: endpoint: Introduce configfs entry for configuring EP functions

Introduce a new configfs entry to configure the EP function (like
configuring the standard configuration header entries) and to bind the EP
function with EP controller.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>