[S390] sclp_vt220: speed up console output for interactive work

Currently an output buffer can wait up to HZ/2 until the buffer is
flushed. The wait time is noticeable in interactive tools like mc.

Change the value to HZ/20, which seems enough for interactive work.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>