Fix the return type of mvebu_pcie_map_registers in the
error path to have __iomem on it. Fixes the following
sparse warning:

drivers/pci/controller/pci-mvebu.c:716:31: warning: incorrect type in return expression (different address spaces)
drivers/pci/controller/pci-mvebu.c:716:31:    expected void [noderef] <asn:2> *
drivers/pci/controller/pci-mvebu.c:716:31:    got void *

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Andrew Murray <andrew.murray@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: linux-pci@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org