[prefix] Ignore PCI autoboot device location if set to 00:00.0

qemu can load an option ROM which is not associated with a particular
PCI device using the "-option-rom" syntax.  Under these circumstances,
we should ignore the PCI bus:dev.fn address that we expect to find in
%ax on entry to the initialisation vector.

Fix by using the PCI bus:dev.fn address only if it is non-zero.  Since
00:00.0 will always be the host bridge, it can never be the address of
a network card.

Reported-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>