net: arc_emac: mark arc_mdio_reset() static

We get 1 warning when building kernel with W=1:
drivers/net/ethernet/arc/emac_mdio.c:107:5: warning: no previous prototype for 'arc_mdio_reset' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.

Signed-off-by: Baoyou Xie <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/arc/emac_mdio.c

···
  * @bus: points to the mii_bus structure
  * Description: reset the MII bus
  */
-int arc_mdio_reset(struct mii_bus *bus)
+static int arc_mdio_reset(struct mii_bus *bus)
 {
 	struct arc_emac_priv *priv = bus->priv;
 	struct arc_emac_mdio_bus_data *data = &priv->bus_data;