s390: make calls to TRACE_IRQS_OFF/TRACE_IRQS_ON balanced

In case of udelay CIF_IGNORE_IRQ is set. This leads to an unbalanced
call of TRACE_IRQS_OFF and TRACE_IRQS_ON. That is: from lockdep's
point of view TRACE_IRQS_ON is called one time too often.

This doesn't fix any real bug, just makes the calls balanced.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>