ramips: correct R6220 button flag

All buttons on the Netgear R6220 are active-low while they are flagged
as active-high.

The GPIO status reads the following for no buttons pressed:

root@64367-r6220:~# cat /sys/kernel/debug/gpio
gpio-7   (                    |wps                 ) in  hi
gpio-8   (                    |wifi                ) in  hi
gpio-14  (                    |reset               ) in  hi

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit f7f9fe5256ebb660d3160452c3c01a9eb080938f)

target/linux/ramips/dts/R6220.dtsi

@@ -58,19 +58,19 @@
 
 		wps {
 			label = "wps";
-			gpios = <&gpio0 7 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpio0 7 GPIO_ACTIVE_LOW>;
 			linux,code = <KEY_WPS_BUTTON>;
 		};
 
 		wifi {
 			label = "wifi";
-			gpios = <&gpio0 8 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpio0 8 GPIO_ACTIVE_LOW>;
 			linux,code = <KEY_RFKILL>;
 		};
 
 		reset {
 			label = "reset";
-			gpios = <&gpio0 14 GPIO_ACTIVE_HIGH>;
+			gpios = <&gpio0 14 GPIO_ACTIVE_LOW>;
 			linux,code = <KEY_RESTART>;
 		};
 	};