net: phy: allow a PHY driver to define neither features nor get_features

Meanwhile we have generic functions for reading the abilities of
Clause 22 / 45 PHY's. This allows to use them as fallback in case
callback get_features isn't set. Benefit is the reduction of
boilerplate code in PHY drivers.

v2:
- adjust the comment in phy_driver_register to match the code

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>