PCI: Make pcim_iounmap_region() a public function

PCI: Make pcim_iounmap_region() a public function

The function pcim_iounmap_regions() is problematic because it uses a
bitmask mechanism to release / iounmap multiple BARs at once. It, thus,
prevents getting rid of the problematic iomap table mechanism which was
deprecated in commit e354bb84a4c1 ("PCI: Deprecate pcim_iomap_table(),
pcim_iomap_regions_request_all()").

pcim_iounmap_region() does not have that problem. Make it public as the
successor of pcim_iounmap_regions().

Link: https://lore.kernel.org/r/20241016094911.24818-3-pstanner@redhat.com
Signed-off-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>