i40e: fix invalid void return in FCoE code

A function was calling i40e_tx_map with return, but tx_map returns
void, and the caller returns void, so just drop the return, and
everything is good.

Change-ID: I53fc676d517864761e7cbb8ca83f1ef0c15b1f8f
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>