PCI: mvebu: Return zero for reserved or unimplemented config space

PCI requires reads to reserved or unimplemented configuration space to
return zero and complete normally (see PCI r3.0, sec 6.1).  However, the
root port software implementation was returning 0xfffffff and
PCIBIOS_BAD_REGISTER_NUMBER.

Return zero when reading reserved or unimplemented config space.

[bhelgaas: changelog]
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com> (Armada XP GP)
Tested-by: Andrew Lunn <andrew@lunn.ch> (Kirkwood DIR665)
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

drivers/pci/host/pci-mvebu.c

@@ -515,8 +515,13 @@ static int mvebu_sw_pci_bridge_read(struct mvebu_pcie_port *port,
 		break;
 
 	default:
-		*value = 0xffffffff;
-		return PCIBIOS_BAD_REGISTER_NUMBER;
+		/*
+		 * PCI defines configuration read accesses to reserved or
+		 * unimplemented registers to read as zero and complete
+		 * normally.
+		 */
+		*value = 0;
+		return PCIBIOS_SUCCESSFUL;
 	}
 
 	if (size == 2)