ath79: convert remaining mtd-mac-address-increment

Commit d284e6ef ("treewide: convert mtd-mac-address-increment* to
generic implementation") renamed "mtd-mac-address-increment" property
to "mac-address-increment". Convert remaining usages that have been
added after that.

Fixes: af8a059b ("ath79: add support for GL.iNet GL-XE300")
Signed-off-by: Sungbo Eo <mans0n@gorani.run>

target/linux/ath79/dts/qca9531_glinet_gl-xe300.dts

@@ -143,7 +143,7 @@
 
 	nvmem-cells = <&macaddr_art_0>;
 	nvmem-cell-names = "mac-address";
-	mtd-mac-address-increment = <1>;
+	mac-address-increment = <1>;
 };
 
 &eth1 {