KVM: PPC: fix comparison to bool warning

Fix the following coccicheck warning:

./arch/powerpc/kvm/booke.c:503:6-16: WARNING: Comparison to bool
./arch/powerpc/kvm/booke.c:505:6-17: WARNING: Comparison to bool
./arch/powerpc/kvm/booke.c:507:6-16: WARNING: Comparison to bool

Reported-by: Tosk Robot <tencent_os_robot@tencent.com>
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Acked-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1604764178-8087-1-git-send-email-kaixuxia@tencent.com