[PPC] 8xx: swap bug-fix

This makes swap routines operate correctly on the ppc_8xx based machines.

Recent kernel's size makes swap feature very important on low-memory platfor
those are actually non-operable without it.

Signed-off-by: Yuri Tikhonov <yur@emcraft.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>