powerpc: Fix incorrect setting of __HAVE_ARCH_PTE_SPECIAL

[I'm going to fix upstream differently, by having all CPU types
actually support _PAGE_SPECIAL, but I prefer the simple and obvious
fix for -stable. -- Ben]

The test that decides whether to define __HAVE_ARCH_PTE_SPECIAL on
powerpc is bogus and will end up always defining it, even when
_PAGE_SPECIAL is not supported (in which case it's 0) such as on
8xx or 40x processors.

Signed-off-by: Bernhard Weirich <bernhard.weirich@riedel.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>