net/i40e: fix unchecked Tx cleanup error

net/i40e: fix unchecked Tx cleanup error

Coverity complains of unchecked return value warning of
i40e_xmit_cleanup, while this cleanup is opportunistic and will not
cause problems if it fails. So instead of checking the return value of
i40e_xmit_cleanup and return in case of cleanup failure, we directly
cast it to void function to make the Coverity happy.

Coverity issue: 353617
Fixes: 4861cde46116 ("i40e: new poll mode driver")
Cc: stable@dpdk.org
Signed-off-by: Beilei Xing <beilei.xing@intel.com>
Acked-by: Xiaolong Ye <xiaolong.ye@intel.com>