alpha/PCI: Call iomem_is_exclusive() for IORESOURCE_MEM, but not IORESOURCE_IO

The alpha pci_mmap_resource() is used for both IORESOURCE_MEM and
IORESOURCE_IO resources, but iomem_is_exclusive() is only applicable for
IORESOURCE_MEM.

Call iomem_is_exclusive() only for IORESOURCE_MEM resources, and do it
earlier to match the generic version of pci_mmap_resource().

Fixes: 10a0ef39 ("PCI/alpha: pci sysfs resources")
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Ivan Kokshaysky <ink@jurassic.park.msu.ru>