Rather than returning ~0, return 0 for every register in case of failure
mode. The '0' is better to indicate that there's no device there. It avoids
SeaBIOS detecting a device and getting stuck on it trying to read and write
its registers.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>