Fix __iomem attribute on msg variable passed to readl() in
the decode_msi_hwirq() function. Fixes the following sparse
warning:

drivers/pci/controller/pcie-iproc-msi.c:301:17: warning: incorrect type in argument 1 (different address spaces)
drivers/pci/controller/pcie-iproc-msi.c:301:17:    expected void const volatile [noderef] <asn:2> *addr
drivers/pci/controller/pcie-iproc-msi.c:301:17:    got unsigned int [usertype] *[assigned] msg

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Andrew Murray <andrew.murray@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Ray Jui <rjui@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Cc: bcm-kernel-feedback-list@broadcom.com
Cc: linux-pci@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org