powerpc/xive: Assign boolean values to a bool variable

Fix the following coccicheck warnings:

./arch/powerpc/kvm/book3s_xive.c:1856:2-17: WARNING: Assignment of 0/1
to bool variable.

./arch/powerpc/kvm/book3s_xive.c:1854:2-17: WARNING: Assignment of 0/1
to bool variable.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1612680192-43116-1-git-send-email-jiapeng.chong@linux.alibaba.com