um: virt-pci: fix platform map offset

The offset is currently always zero so the backend can't distinguish
between accesses to different ioremapped areas.

Fixes: 522c532c ("virt-pci: add platform bus support")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/virt-pci.c

@@ -971,7 +971,7 @@ static long um_pci_map_platform(unsigned long offset, size_t size,
 	*ops = &um_pci_device_bar_ops;
 	*priv = &um_pci_platform_device->resptr[0];
 
-	return 0;
+	return offset;
 }
 
 static const struct logic_iomem_region_ops um_pci_platform_ops = {