ethtool: allow netdev driver to define phy tunables

Define get/set phy tunable callbacks in ethtool ops.
This will allow MAC drivers with integrated PHY still to implement
these tunables.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>