Avoid false positive warnings like this with gcc 7.1:

drivers/s390/cio/ccwgroup.c:41:21:
 warning: '%d' directive writing between 1 and 10 bytes into a region of size 4
   sprintf(str, "cdev%d", i);

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>