[pxe] Avoid touching fields that may not exist in PXENV_UNDI_GET_NIC_TYPE

Earlier versions of the PXE specification do not have the SubVendor_ID
and SubDevice_ID fields, and some NBPs may not provide space for them.
Avoid overwriting the contents of these fields, just in case.

This is similar to the problem with the BufferLimit field in
PXENV_GET_CACHED_INFO.

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

src/arch/i386/interface/pxe/pxe_undi.c

 		info->Sub_Class = PCI_SUB_CLASS ( dev->desc.class );
 		info->Prog_Intf = PCI_PROG_INTF ( dev->desc.class );
 		info->BusDevFunc = dev->desc.location;
-		/* Cheat: remaining fields are probably unnecessary,
-		 * and would require adding extra code to pci.c.
+		/* Earlier versions of the PXE specification do not
+		 * have the SubVendor_ID and SubDevice_ID fields.  It
+		 * is possible that some NBPs will not provide space
+		 * for them, and so we must not fill them in.
 		 */
-		undi_get_nic_type->info.pci.SubVendor_ID = 0xffff;
-		undi_get_nic_type->info.pci.SubDevice_ID = 0xffff;
-		DBG ( " PCI %02x:%02x.%x %04x:%04x (%04x:%04x) %02x%02x%02x "
+		DBG ( " PCI %02x:%02x.%x %04x:%04x ('%04x:%04x') %02x%02x%02x "
 		      "rev %02x\n", PCI_BUS ( info->BusDevFunc ),
 		      PCI_SLOT ( info->BusDevFunc ),
 		      PCI_FUNC ( info->BusDevFunc ), info->Vendor_ID,