Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: i386):

drivers/net/wan/sdla.c: In function ‘sdla_errors’:
drivers/net/wan/sdla.c:414:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (cmd == SDLA_INFORMATION_WRITE)
       ^
drivers/net/wan/sdla.c:417:3: note: here
   default:
   ^~~~~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>