prism54: isl_ioctl: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/intersil/prism54/isl_ioctl.c

@@ -1691,6 +1691,7 @@ static int prism54_get_encodeext(struct net_device *ndev,
 	case DOT11_AUTH_BOTH:
 	case DOT11_AUTH_SK:
 		wrqu->encoding.flags |= IW_ENCODE_RESTRICTED;
+		/* fall through */
 	case DOT11_AUTH_OS:
 	default:
 		wrqu->encoding.flags |= IW_ENCODE_OPEN;