Newer BOFM builds provide support for mapping multiple physical ports
to a single PCI bus:dev.fn via PCI VPD descriptions.  These builds
will also leave the {slot,port} field intact, and will populate the
mport field with a meaningful value.

Older BOFM builds will zero out the {slot,port} field.  A zero value
in this field may indicate either a genuine zero value (i.e. slot 0
first port) or an older build.

Signed-off-by: Michael Brown <mcb30@ipxe.org>