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>

arch/powerpc/kvm/book3s_64_slb.S

@@ -113,7 +113,7 @@ slb_do_enter:
 
 	/* Remove all SLB entries that are in use. */
 
-	li	r0, r0
+	li	r0, 0
 	slbmte	r0, r0
 	slbia