[MIPS] RTLX: Harden against compiler reordering and optimization.

RTLX communication is based on lock-free shared memory buffers.  It
happened to be working by luck so far but relies on the optimizer doing
certain optimizations but no reordering.

Fixed by inserting proper barriers in rtlx_read and rtlx_write, and careful
pointer dereferencing.

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