ixgbe: add check for supported modes

When setting advertised speed/duplex with ethtool.

Also cleaned up the comment since we also support 100/F.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>