ixgbe: Support 2.5Gb and 5Gb speed

Though not advertised through ethtool, if the link partner advertises a
2.5Gb or 5Gb connection, and the adapter supports it, allow the speed to be
used.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>