net: phy: allow EEE with any interface mode

EEE is able to work in any PHY interface mode, there is nothing which
fundamentally restricts it to only a few modes.  For example, EEE works
in SGMII mode with the Marvell 88E1512.

Rather than just adding SGMII mode to the list, Florian suggests
removing the list of interface modes entirely:

  It actually sounds like we should just kill the check entirely,
  it does not appear that any of the interface mode would not
  fundamentally be able to support EEE, because the "lowest" mode
  we support is MII, and even there it's quite possible to support
  EEE.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>