block: DAC960: shut up format-overflow warning

block: DAC960: shut up format-overflow warning

gcc-7 points out that a large controller number would overflow the
string length for the procfs name and the firmware version string:

drivers/block/DAC960.c: In function 'DAC960_Probe':
drivers/block/DAC960.c:6591:38: warning: 'sprintf' may write a terminating nul past the end of the destination [-Wformat-overflow=]
drivers/block/DAC960.c: In function 'DAC960_V1_ReadControllerConfiguration':
drivers/block/DAC960.c:1681:40: error: '%02d' directive writing between 2 and 3 bytes into a region of size between 2 and 5 [-Werror=format-overflow=]
drivers/block/DAC960.c:1681:40: note: directive argument in the range [0, 255]
drivers/block/DAC960.c:1681:3: note: 'sprintf' output between 10 and 14 bytes into a destination of size 12

Both of these seem appropriately sized, and using snprintf()
instead of sprintf() improves this by ensuring that even
incorrect data won't cause undefined behavior here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>