phy: mdio: add kerneldoc for __devm_mdiobus_register()

This function is not documented. Add a short kerneldoc description.
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/mdio_devres.c

@@ -2,6 +2,13 @@
 
 #include <linux/phy.h>
 
+/**
+ * __devm_mdiobus_register - Resource-managed variant of mdiobus_register()
+ * @bus:	MII bus structure to register
+ * @owner:	Owning module
+ *
+ * Returns 0 on success, negative error number on failure.
+ */
 int __devm_mdiobus_register(struct mii_bus *bus, struct module *owner)
 {
 	int ret;