scsi: Convert a strncmp() call into a strcmp() call

This patch avoids that smatch reports the following warning:

drivers/scsi/scsi_sysfs.c:117: check_set() error: strncmp() '"-"' too small (2 vs 20)

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>