KVM: PPC: Book3S: Fix gas warning due to using r0 as immediate 0

This fixes the message:

arch/powerpc/kvm/book3s_segment.S: Assembler messages:
arch/powerpc/kvm/book3s_segment.S:330: Warning: invalid register expression

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>