ixgbe: ethtool support to change advertised link modes of 82599 adapters

Add ethtool support to change advertised link modes/autoneg settings of
82599 multispeed fiber adapters.

Signed-off-by: Mallikarjuna R Chilakala <mallikarjuna.chilakala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>