net/iavf: fix unchecked Tx cleanup error

net/iavf: fix unchecked Tx cleanup error

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

Coverity issue: 363045
Fixes: 02d212ca3125 ("net/iavf: rename remaining avf strings")
Cc: stable@dpdk.org
Signed-off-by: Leyi Rong <leyi.rong@intel.com>
Acked-by: Qi Zhang <qi.z.zhang@intel.com>