igb: make igb_set_fc_watermarks() return void

This function always return 0 now, we can make it return void to
simplify the code. This fixes the following coccicheck warning:

drivers/net/ethernet/intel/igb/e1000_mac.c:728:5-12: Unneeded variable:
"ret_val". Return "0" on line 751

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>