include: mdio.h: Make bus functions optional

Make bus_enable and bus_disable optional for drivers
to implement by checking if it is NULL in the api struct.

Signed-off-by: Declan Snyder <[email protected]>

Author: decsny

include/zephyr/drivers/mdio.h

@@ -73,7 +73,9 @@ static inline void z_impl_mdio_bus_enable(const struct device *dev)
 	const struct mdio_driver_api *api =
 		(const struct mdio_driver_api *)dev->api;
 
-	api->bus_enable(dev);
+	if (api->bus_enable != NULL) {
+		api->bus_enable(dev);
+	}
 }
 
 /**
@@ -89,7 +91,9 @@ static inline void z_impl_mdio_bus_disable(const struct device *dev)
 	const struct mdio_driver_api *api =
 		(const struct mdio_driver_api *)dev->api;
 
-	api->bus_disable(dev);
+	if (api->bus_disable != NULL) {
+		api->bus_disable(dev);
+	}
 }
 
 /**