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.comReported-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>