ixgbe: Adding 100MB FULL support in ethtool

Current driver does not show 100MB support in ethtool.
Adding support for the same.

Signed-off-by: Atita Shirwaikar <atita.shirwaikar@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>