pci: Replace unneeded type casts in calls of pci_register_bar

There is no need for these type casts (as other existing
code shows). So re-write the first argument without
type cast (and remove a related TODO comment).

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>