PCI: Fail pci_ioremap_bar() on unassigned resources

Make pci_ioremap_bar() fail if we're trying to map a BAR that hasn't been
assigned.

Normally pci_enable_device() will fail if a BAR hasn't been assigned, but a
driver can successfully call pci_enable_device_io() even if a memory BAR
hasn't been assigned.  That driver should not be able to use
pci_ioremap_bar() to map that unassigned memory BAR.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>