PCI MSI: MSI-X cleanup, msix_map_region()

Cleanup based on the prototype from Matthew Milcox.

Reviewed-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>