Simplify the function, no functional change intended.

- Remove not needed variable unsupp, I think code is even better
  readable now.
- Move setting phydev->eee_enabled out of the if clause
- Simplify return value handling
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/442277c7-7431-4542-80b5-1d3d691714d7@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>