net: phy: bcm87xx: improve bcm87xx_config_init and feature detection

PHY drivers don't have to and shouldn't fiddle with phylib internals.
Most of the code in bcm87xx_config_init() can be removed because
phylib takes care.

In addition I replaced usage of PHY_10GBIT_FEC_FEATURES with an
implementation of the get_features callback. PHY_10GBIT_FEC_FEATURES
is used by this driver only and it's questionable whether there
will be any other PHY supporting this mode only. Having said that
in one of the next kernel versions we may decide to remove it.

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