net: phy: core: use genphy version of callbacks read_status and config_aneg per default

read_status and config_aneg are the only mandatory callbacks and most
of the time the generic implementation is used by drivers.
So make the core fall back to the generic version if a driver doesn't
implement the respective callback.

Also currently the core doesn't seem to verify that drivers implement
the mandatory calls. If a driver doesn't do so we'd just get a NPE.
With this patch this potential issue doesn't exit any longer.

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