ath79: fix TP-Link Archer C7 v2 wlan1 MAC address

The mac address for the 2.4 wireless need to be decremented by one.

Correct MAC adress increments for this board are:
wlan0 (5GHz)   : -2
wlan1 (2.4GHz) : -1
eth1  (LAN)    :  0
eth0  (WAN)    :  1

Signed-off-by: Aleksandr V. Piskunov <aleksandr.v.piskunov@gmail.com>

target/linux/ath79/dts/qca9558_tl-archer-c7.dtsi

@@ -224,4 +224,5 @@
 	status = "okay";
 	mtd-cal-data = <&art 0x1000>;
 	mtd-mac-address = <&uboot 0x1fc00>;
+	mtd-mac-address-increment = <(-1)>;
 };