hamradio: dmascc: avoid -Wformat-overflow warning

gcc warns that the device name might overflow:

drivers/net/hamradio/dmascc.c: In function 'dmascc_init':
drivers/net/hamradio/dmascc.c:584:22: error: 'sprintf' may write a terminating nul past the end of the destination [-Werror=format-overflow=]
   sprintf(dev->name, "dmascc%i", 2 * n + i);
drivers/net/hamradio/dmascc.c:584:3: note: 'sprintf' output between 8 and 17 bytes into a destination of size 16
   sprintf(dev->name, "dmascc%i", 2 * n + i);

>From the static data in this file, I can tell that the index is
strictly limited to 16, so it won't overflow. This simply changes
the sprintf() to snprintf(), which is a good idea in general,
and shuts up this warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>