i40e: move i40e_xsk_umem function

The i40e_xsk_umem function was explicitly inlined in i40e.h. There is
no reason for that, so move it to i40e_main.c instead.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>