net: wimax/i2400m: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch
cases where we are expecting to fall through.

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/wimax/i2400m/usb.c

@@ -210,6 +210,7 @@ int __i2400mu_send_barker(struct i2400mu *i2400mu,
 			msleep(10);	/* give the device some time */
 			goto retry;
 		}
+		/* fall through */
 	case -EINVAL:			/* while removing driver */
 	case -ENODEV:			/* dev disconnect ... */
 	case -ENOENT:			/* just ignore it */