[netdevice] Fix erroneous use of free(iobuf) instead of free_iob(iobuf)

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

src/net/netdevice.c

 
 			/* Discard first deferred packet */
 			list_del ( &iobuf->list );
-			free ( iobuf );
+			free_iob ( iobuf );
 
 			/* Report discard */
 			discarded++;