net: stmmac: fix EEE init issue when paired with EEE capable PHYs

When STMMAC is paired with Energy-Efficient Ethernet(EEE) capable PHY,
and the PHY is advertising EEE by default, we need to enable EEE on the
xPCS side too, instead of having user to manually trigger the enabling
config via ethtool.

Fixed this by adding xpcs_config_eee() call in stmmac_eee_init().

Fixes: 7617af3d ("net: pcs: Introducing support for DWC xpcs Energy Efficient Ethernet")
Cc: Michael Sit Wei Hong <michael.wei.hong.sit@intel.com>
Signed-off-by: Wong Vee Khee <vee.khee.wong@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>