[POWERPC] Kill sparse warning in HPTE_V_COMPARE()

Fixes sparse warning: constant 0xffffffffffffff80 is so big it is
unsigned long

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>