[POWERPC] Fix __set_fixmap() for STRICT_MM_TYPECHECKS

__set_fixmap() in pgtable_32.c currently fails to compile if
STRICT_MM_TYPECHECKS is defined.  This fixes it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>