pci: Properly configure prefetchable memory region

Forcibly set hose->pci_prefetch to NULL to make sure it will be setup.
This will help if for any reason callers didn't make sure themselves to
NULL the field.

Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/pci/pci_auto.c

@@ -297,7 +297,7 @@ void pciauto_config_init(struct pci_controller *hose)
 {
 	int i;
 
-	hose->pci_io = hose->pci_mem = NULL;
+	hose->pci_io = hose->pci_mem = hose->pci_prefetch = NULL;
 
 	for (i = 0; i < hose->region_count; i++) {
 		switch(hose->regions[i].flags) {