i40e: Set fec_config when forcing link state

This patch configures FEC setting in i40e_force_link_state().
For some reason setting this field was overlooked thus causing
25G link to be configured incorrectly.

Signed-off-by: Mariusz Stachura <mariusz.stachura@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>