s390/vmlogrdr: fix array access in vmlogrdr_open()

Fix check within vmlogrdr_open() if the minor address is not larger
than the number of array elements.

Found with "smatch":

drivers/s390/char/vmlogrdr.c:318 vmlogrdr_open() warn:
  buffer overflow 'sys_ser' 3 <= 3

Acked-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>