s390/irq: Set CIF_NOHZ_DELAY in do_io_irq()

Both do_airq_interrupt() and do_io_interrupt() set
CIF_NOHZ_DELAY. Move it to do_io_irq() to simplify
the code.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>