ath79: Fix system LED on TP-Link WR740/741 v4

The system LED on these devices is ACTIVE_LOW, change back to what
it is on ar71xx.

Signed-off-by: Paul Wassi <[email protected]>

target/linux/ath79/dts/ar9331_tplink_tl-wr741nd-v4.dtsi

@@ -69,7 +69,7 @@
 
 		system: system {
 			label = "tp-link:green:system";
-			gpios = <&gpio 27 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpio 27 GPIO_ACTIVE_LOW>;
 		};
 
 		wan {