A compiler barrier (explicit on the read side, implicit on the write
side) is not quite enough for what has to be accomplished here.  Use
hardware memory barriers on systems which need them.

(Of course a full fix of generation handling would require much more
than this.  The ieee1394 core's bus generation counter had to be tied to
the controller's bus generation counter; cf. Kristian's stack.  It's
just that I have other current business with the code around these
barrier()s, so why not do at least this small fix.)

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>