ath79: ubnt-bullet-m-xw: fix Ethernet PHY traffic

Since commit 6f2e1b74 ("ath79: disable delays on AT803X config init")
Ubiquiti XW boards equipped with AR8035 PHY suffered from lack of
outbound traffic on the Ethernet port. This was caused by the fact, the
U-boot has set this during boot and it wasn't reset by the PHY driver,
and the corresponding setting in device tree was wrong.

Set the 'phy-mode = "rgmii-txid"' at the &eth0, and drop this property
from PHY node, as it is not parsed there. This causes the device to
connect using Ethernet once again.

Fixes: db4b6535 ("ath79: Add support for Ubiquity Bullet M (XW)")
Fixes: 6f2e1b74 ("ath79: disable delays on AT803X config init")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
(cherry picked from commit a9b2ba4d7ba06e1ac4ff7da3eb9b4038b94d9fbb)

target/linux/ath79/dts/ar9342_ubnt_bullet-m-xw.dts

@@ -11,7 +11,6 @@
 	status = "okay";
 
 	phy4: ethernet-phy@4 {
-		phy-mode = "rgmii";
 		reg = <4>;
 		max-speed = <100>;
 	};
@@ -20,7 +19,7 @@
 &eth0 {
 	status = "okay";
 
-	phy-mode = "rgmii";
+	phy-mode = "rgmii-txid";
 	phy-handle = <&phy4>;
 
 	gmac-config {