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>