net: miiphy: Introduce mdio_init()

Introduce mdio_init() split off from mdio_alloc(), which is used
to initialize already allocated struct mii_dev.

Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/include/miiphy.h b/include/miiphy.h
index 0464f5d..40eb146 100644
--- a/include/miiphy.h
+++ b/include/miiphy.h
@@ -44,6 +44,7 @@
 
 void miiphy_listdev(void);
 
+void mdio_init(struct mii_dev *bus);
 struct mii_dev *mdio_alloc(void);
 void mdio_free(struct mii_dev *bus);
 int mdio_register(struct mii_dev *bus);