net: wan: sdla: Mark expected switch fall-through

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>

drivers/net/wan/sdla.c

@@ -413,6 +413,7 @@ static void sdla_errors(struct net_device *dev, int cmd, int dlci, int ret, int
 		case SDLA_RET_NO_BUFS:
 			if (cmd == SDLA_INFORMATION_WRITE)
 				break;
+			/* Else, fall through */
 
 		default: 
 			netdev_dbg(dev, "Cmd 0x%02X generated return code 0x%02X\n",