MIPS: Octeon: Fix warning.

Cong Ding <dinggnu@gmail.com> reports correctly that the variable dummy
is being used without initialization.  That said, I can't reproduce this
warning with GCC 4.7.1.  However, since the variable dummy servces no
real purpose, I'm going for a different fix.  This fix
includes https://patchwork.linux-mips.org/patch/4801/ plus Geert's
suggestion to use ACCESS_ONCE().

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>