net: phy: genphy: Allow overwriting features

of_set_phy_supported allows overwiting hardware capabilities of
a phy with values from the devicetree. This does not work with
the genphy driver though because the genphys config_init function
will overwrite all values adjusted by of_set_phy_supported. Fix
this by initialising the genphy features in the phy_driver struct
and in config_init just limit the features to the ones the hardware
can actually support. The resulting features are a subset of the
devicetree specified features and the hardware features.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>