s390/tape: fix stringop-truncation gcc 8 warnings

Use strlcpy to make sure strings are NUL-terminated. Fixes the following
gcc 8 warning:

inlined from 'register_tape_dev' at drivers/s390/char/tape_class.c:57:2:
./include/linux/string.h:246:9: warning: '__builtin_strncpy' specified
bound 32 equals destination size [-Wstringop-truncation]

Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>