s390/qdio: increase string buffer size

Avoid false positive warnings like this with gcc 7.1:

drivers/s390/cio/qdio_debug.h:63:4:
 note: 'snprintf' output between 8 and 17 bytes into a destination of size 16
    snprintf(debug_buffer, QDIO_DBF_LEN, text);

and simply increase the size of the string buffer.

Reviewed-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>