ramips: fix ZyXEL NWA55AXE model name

The model name was missing a letter.
Signed-off-by: NDavid Bauer <mail@david-bauer.net>

target/linux/ramips/dts/mt7621_zyxel_nwa55axe.dts

@@ -2,5 +2,5 @@
 
 / {
 	compatible = "zyxel,nwa55axe", "mediatek,mt7621-soc";
-	model = "ZyXEL NWA55AX";
+	model = "ZyXEL NWA55AXE";
 };