ath79: flag FritzBox 4020 buttons as active low

Buttons of AVM FritzBox 4020 are incorrectly flagged as active high.

This was an oversight as RFKill button was working as expected even
with incorrectly flagged GPIO.

Signed-off-by: David Bauer <mail@david-bauer.net>

target/linux/ath79/dts/qca9561_avm_fritz4020.dts

@@ -96,13 +96,13 @@
 		wps_button {
 			label = "WPS button";
 			linux,code = <KEY_WPS_BUTTON>;
-			gpios = <&gpio 2 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpio 2 GPIO_ACTIVE_LOW>;
 		};
 
 		wlan_button {
 			label = "WLAN button";
 			linux,code = <KEY_RFKILL>;
-			gpios = <&gpio 21 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpio 21 GPIO_ACTIVE_LOW>;
 		};
 	};
 };