net: usb: r8152: Use linkmode helpers for EEE

Make use of the existing linkmode helpers for converting PHY EEE
register values into links modes, now that ethtool_keee uses link
modes, rather than u32 values.

Rework determining if EEE is active to make is similar as to how
phylib decides, and make use of a phylib helper to validate if EEE is
valid in for the current link mode. This then requires that PHYLIB is
selected.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>