scsi: qedi: Fix a potential buffer overflow

Tell snprintf() to store at most 255 characters in the output buffer
instead of 256. This patch avoids that smatch reports the following
warning:

drivers/scsi/qedi/qedi_main.c:891: qedi_get_boot_tgt_info() error: snprintf() is printing too much 256 vs 255

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: <QLogic-Storage-Upstream@cavium.com>
Cc: <stable@vger.kernel.org>
Acked-by: Nilesh Javali <nilesh.javali@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>