igc: Remove serdes comments from a description of methods

Serdes interface is not applicable for i225 devices.
Remove this from comments and make comments more clearly.

Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>