scsi: megaraid: fix format-overflow warning

gcc-7 complains that the firmware version strings might overflow
for some values:

drivers/scsi/megaraid.c: In function 'megaraid_probe_one':
drivers/scsi/megaraid.c:314:33: error: '%d' directive writing between 1 and 2 bytes into a region of size between 1 and 2 [-Werror=format-overflow=]
drivers/scsi/megaraid.c:314:33: note: directive argument in the range [0, 15]
drivers/scsi/megaraid.c:314:3: note: 'sprintf' output between 7 and 9 bytes into a destination of size 7
drivers/scsi/megaraid.c:320:35: error: '%d' directive writing between 1 and 2 bytes into a region of size between 1 and 2 [-Werror=format-overflow=]
drivers/scsi/megaraid.c:320:35: note: directive argument in the range [0, 15]
drivers/scsi/megaraid.c:320:3: note: 'sprintf' output between 7 and 9 bytes into a destination of size 7

This makes the code use a truncating snprintf() instead, which shuts
up that warning.

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