net: phy: add generic ndo_do_ioctl handler phy_do_ioctl

A number of network drivers has the same glue code to use phy_mii_ioctl
as ndo_do_ioctl handler. So let's add such a generic ndo_do_ioctl
handler to phylib.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>