s390/dasd: fix gcc 8 stringop-truncation warning

drivers/s390/block/dasd_alias.c:711:2: warning: 'strncpy' output truncated
before terminating nul copying 4 bytes from a string of the same length
[-Wstringop-truncation]
  strncpy((char *) &cqr->magic, "ECKD", 4);

Replace strncpy to int as a buffer with memcpy.

Acked-by: Stefan Haberland <sth@linux.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>