[S390] smp: fix sigp stop handling

According to the architecture a cpu must not necessarily enter stopped
state after completion of a sigp instruction with "stop" order code.
So remove the BUG() statement after self sending sigp stop to avoid
that it ever gets reached.
Also add a sigp busy check to make sure that the order gets delivered.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/kernel/swsusp_asm64.S b/arch/s390/kernel/swsusp_asm64.S
index 7c8653e..0f4ef3b 100644
--- a/arch/s390/kernel/swsusp_asm64.S
+++ b/arch/s390/kernel/swsusp_asm64.S
@@ -199,6 +199,7 @@
 	brc	2,4b			/* busy, try again */
 5:
 	sigp	%r9,%r2,__SIGP_STOP	/* stop resume (current) CPU */
+	brc	2,5b			/* busy, try again */
 6:	j	6b
 
 restart_suspend: