ath79: ubnt,bullet-m-xw: set PHY max-speed to 100Mbps

Onboard AR8035 PHY supports 1000Base-T operation, but onboard
Ethernet magnetics do not. Reduce advertised link speeds to 100Mbps and
lower.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
(cherry picked from commit d406777fb14c84e82f51bab059631af70cf6d5c9)

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

@@ -14,6 +14,7 @@
 	phy4: ethernet-phy@4 {
 		phy-mode = "rgmii";
 		reg = <4>;
+		max-speed = <100>;
 	};
 };