Documentation: fix incorrect macro referencing in mscc-phy-vsc8531.txt

Modify LINK_1000_ACTIVITY and LINK_100_ACTIVITY to VSC8531_LINK_1000_ACTIVITY
and VSC8531_LINK_100_ACTIVITY respectively in the example of ethernet-phy node
according to include/dt-bindings/net/mscc-phy-vsc8531.h.

Signed-off-by: Baisheng Gao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/net/mscc-phy-vsc8531.txt

···
                 compatible = "ethernet-phy-id0007.0570";
                 vsc8531,vddmac		= <3300>;
                 vsc8531,edge-slowdown	= <7>;
-                vsc8531,led-0-mode	= <LINK_1000_ACTIVITY>;
-                vsc8531,led-1-mode	= <LINK_100_ACTIVITY>;
+                vsc8531,led-0-mode	= <VSC8531_LINK_1000_ACTIVITY>;
+                vsc8531,led-1-mode	= <VSC8531_LINK_100_ACTIVITY>;
 		load-save-gpios		= <&gpio 10 GPIO_ACTIVE_HIGH>;
         };