[S390] ftrace: disable tracing on idle psw

Disable tracing on idle psw. Otherwise it would give us huge
preempt off times for idle. Which is rather pointless.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>