of: mdio: Add missing inline to of_mdiobus_child_is_phy() dummy

of: mdio: Add missing inline to of_mdiobus_child_is_phy() dummy

If CONFIG_OF_MDIO=n:

    drivers/net/phy/mdio_bus.c:23:
    include/linux/of_mdio.h:58:13: warning: ‘of_mdiobus_child_is_phy’ defined but not used [-Wunused-function]
     static bool of_mdiobus_child_is_phy(struct device_node *child)
		 ^~~~~~~~~~~~~~~~~~~~~~~

Fix this by adding the missing "inline" keyword.

Fixes: 8bd7e465f9d4d3b5 ("of: mdio: export of_mdiobus_child_is_phy")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>