ixgbe: ethtool correctly identify autoneg setting

This patch enables ethtool to correctly identify flow control (pause
frame) auto negotiation, as well as disallow enabling it when it is not
supported. The ixgbe_device_supports_autoneg_fc function is exported and
used for this purpose.

There is also one minor cleanup of the device_supports_autoneg_fc by
removing an unnecessary return statement.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>