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.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>

include/linux/pci-ep-cfs.h

@@ -19,7 +19,7 @@ void pci_ep_cfs_remove_epf_group(struct config_group *group);
 #else
 static inline struct config_group *pci_ep_cfs_add_epc_group(const char *name)
 {
-	return 0;
+	return NULL;
 }
 
 static inline void pci_ep_cfs_remove_epc_group(struct config_group *group)
@@ -28,7 +28,7 @@ static inline void pci_ep_cfs_remove_epc_group(struct config_group *group)
 
 static inline struct config_group *pci_ep_cfs_add_epf_group(const char *name)
 {
-	return 0;
+	return NULL;
 }
 
 static inline void pci_ep_cfs_remove_epf_group(struct config_group *group)