[netdevice] Return ENOENT for an unknown bus type

It is possible for the preloaded UNDI device to end up with no
specified bus type, since it may not be recognised as either a PCI or
an ISAPnP device.  This will result in a bus type value of zero, which
currently results in NULL being treated as a string pointer by
netdev_fetch_bustype().

Fix by returning ENOENT if an unknown bus type is specified.

Reported-by: Todd Stansell <todd@stansell.org>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/netdev_settings.c

 	assert ( desc->bus_type < ( sizeof ( bustypes ) /
 				    sizeof ( bustypes[0] ) ) );
 	bustype = bustypes[desc->bus_type];
-	assert ( bustype != NULL );
+	if ( ! bustype )
+		return -ENOENT;
 	strncpy ( data, bustype, len );
 	return strlen ( bustype );
 }