gt64xxx: fix crash in gt64120_pci_mapping()

The map/unmap code was assymetric - unmap used the local MemoryRegion while
map used isa_mmio_init(), which cannot handle dynamic mappings.

Fix by using isa_mmio_setup() and the local MemoryRegion.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>
