scsi: gdth: avoid buffer overflow warning

gcc notices that we would overflow the buffer for the
inquiry of the product name if we have too many adapters:

drivers/scsi/gdth.c: In function 'gdth_next':
drivers/scsi/gdth.c:2357:29: warning: 'sprintf' may write a terminating nul past the end of the destination [-Wformat-overflow=]
         sprintf(inq.product,"Host Drive  #%02d",t);
                             ^~~~~~~~~~~~~~~~~~~
drivers/scsi/gdth.c:2357:9: note: 'sprintf' output between 16 and 17 bytes into a destination of size 16
         sprintf(inq.product,"Host Drive  #%02d",t);

This won't happen in practice, so just use snprintf to
truncate the string.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>