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>