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