[S390] Small barrier() and cpu_relax() cleanup.

cpu_relax() has barrier() semantics hence there is no need to use both
of them in conjunction in sclp_sync_wait(). Also change cpu_relax()
so it's more obvious that it has barrier semantics.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/char/sclp.c

@@ -433,7 +433,6 @@ sclp_sync_wait(void)
 		    get_clock() > timeout &&
 		    del_timer(&sclp_request_timer))
 			sclp_request_timer.function(sclp_request_timer.data);
-		barrier();
 		cpu_relax();
 	}
 	local_irq_disable();

include/asm-s390/processor.h

@@ -201,8 +201,7 @@ unsigned long get_wchan(struct task_struct *p);
 static inline void cpu_relax(void)
 {
 	if (MACHINE_HAS_DIAG44)
-		asm volatile("diag 0,0,68" : : : "memory");
-	else
+		asm volatile("diag 0,0,68");
 	barrier();
 }