ramips: fix RT-AC57U button level

Both buttons on the RT-AC57U are active-low. Fix the GPIO flag for the
WPS cutton to fix button behavior.
Signed-off-by: NDavid Bauer <mail@david-bauer.net>
(cherry picked from commit 535b0c70)

target/linux/ramips/dts/RT-AC57U.dts

@@ -47,7 +47,7 @@
 
 		wps {
 			label = "wps";
-			gpios = <&gpio1 11 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpio1 11 GPIO_ACTIVE_LOW>;
 			linux,code = <KEY_WPS_BUTTON>;
 			debounce-interval = <60>;
 		};