net: thunder_bgx: avoid format string overflow warning

gcc warns that the temporary buffer might be too small here:

drivers/net/ethernet/cavium/thunder/thunder_bgx.c: In function 'bgx_probe':
drivers/net/ethernet/cavium/thunder/thunder_bgx.c:1020:16: error: '%d' directive writing between 1 and 10 bytes into a region of size between 9 and 11 [-Werror=format-overflow=]
sprintf(str, "BGX%d LMAC%d mode", bgx->bgx_id, lmacid);
             ^~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/cavium/thunder/thunder_bgx.c:1020:16: note: directive argument in the range [0, 2147483647]
drivers/net/ethernet/cavium/thunder/thunder_bgx.c:1020:3: note: 'sprintf' output between 16 and 27 bytes into a destination of size 20

This probably can't happen, but it can't hurt to make it long
enough for the theoretical limit.

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