net: phylink: add phylink_init_eee() helper

Provide phylink_init_eee() to allow MAC drivers to initialise PHY EEE
from within the ethtool set_eee() method.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>