PCI: mvebu: return NULL instead of ERR_PTR(ret)

PCI: mvebu: return NULL instead of ERR_PTR(ret)

Return NULL instead of ERR_PTR(ret) in order to fix the following
sparse warning:

drivers/pci/host/pci-mvebu.c:744:31: warning: incorrect type in return expression (different address
spaces)
drivers/pci/host/pci-mvebu.c:744:31:    expected void [noderef] <asn:2>*
drivers/pci/host/pci-mvebu.c:744:31:    got void *

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>