econet: fix wrong board name for Zyxel PMG5617GA

Board name was specified incorrectly in the DT, fix.
This bug was introduced in #20580 (31f5fc8fea8931f5) which was
merged last week, and econet is still a mostly experimental target,
so it is considered unnecessary to update SUPPORTED_DEVICES.

Signed-off-by: Caleb James DeLisle <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/21023
Signed-off-by: Robert Marko <[email protected]>

target/linux/econet/base-files/sbin/en75_chboot

@@ -129,7 +129,7 @@ main() {
         code_openwrt=30000000
         code_factory=31000000
         ;;
-    tplink,zyxel-pmg5617ga)
+    zyxel,pmg5617ga)
         # 00060fff
         part=$(part_named '"reservearea"')
         offset_blocks=3

target/linux/econet/dts/en751221_zyxel_pmg5617ga.dts

@@ -5,7 +5,7 @@
 
 / {
 	model = "Zyxel PMG5617GA";
-	compatible = "tplink,zyxel-pmg5617ga", "econet,en751221";
+	compatible = "zyxel,pmg5617ga", "econet,en751221";
 
 	memory@0 {
 		device_type = "memory";