i40e: fix possible compiler warning in xsk TX path

With certain gcc versions, it was possible to get the warning
"'tx_desc' may be used uninitialized in this function" for the
i40e_xmit_zc. This was not possible, however this commit simplifies
the code path so that this warning is no longer emitted.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>