[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>

src/arch/i386/core/pci_autoboot.c

  */
 static void pci_autoboot_init ( void ) {
 
-	autoboot_device.bus_type = BUS_TYPE_PCI;
-	autoboot_device.location = autoboot_busdevfn;
+	if ( autoboot_busdevfn ) {
+		autoboot_device.bus_type = BUS_TYPE_PCI;
+		autoboot_device.location = autoboot_busdevfn;
+	}
 }
 
 /** PCI autoboot device initialisation function */