ar71xx: wpj531: fix SIG1/RSS1 LED GPIO

ar71xx: wpj531: fix SIG1/RSS1 LED GPIO

In commit 6c937df749c7 ("ar71xx: wpj531: fix GPIOs for LED") wrong GPIO
13 for SIG1/RSS1 LED was commited, the correct GPIO number for this LED
is 12.

It's listed in "Hardware Guide - wpj531 7A06 (02/07/2019)" as GPIO12/RSS1
on the LED header and same GPIO 12 is used in the vendor's SDK as well.

Fixes: 6c937df749c7 ("ar71xx: wpj531: fix GPIOs for LED")
Signed-off-by: Leon M. George <leon@georgemail.eu>
[commit subject/message facelift]
Signed-off-by: Petr Štetiar <ynezz@true.cz>