[bofm] Report {slot,port} to {bus:dev.fn,mport} mapping on newer BOFM builds
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>