ar71xx: use status led for GL.iNet GL-AR750S

Use power led for device status.

The status led behavior has already been fixed in af28d8a539
("ath79: add support for GL.iNet GL-AR750S") when porting the
device to ath79. This fixes it for ar71xx as well.

Signed-off-by: Jan Alexander <jan@nalx.net>
[minor commit title/message adjustments]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
(cherry picked from commit d394c354ee0e8660f876889f6293803c581cbf85)

target/linux/ar71xx/base-files/etc/diag.sh

@@ -75,6 +75,7 @@ get_status_led() {
 	fritz300e|\
 	fritz4020|\
 	fritz450e|\
+	gl-ar750s|\
 	gl-usb150|\
 	mr12|\
 	mr16|\