igc: check return value of ret_val in igc_config_fc_after_link_up

Check return value from ret_val to make error check actually work.

Fixes: 4eb80801 ("igc: Add setup link functionality")
Signed-off-by: Kevin Lo <kevlo@kevlo.org>
Acked-by: Sasha Neftin <sasha.neftin@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>