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

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

Since commit 6f2e1b7485f0 ("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: db4b6535f837 ("ath79: Add support for Ubiquity Bullet M (XW)")
Fixes: 6f2e1b7485f0 ("ath79: disable delays on AT803X config init")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>