net/fm10k: fix Tx prepare to set positive rte_errno
net/fm10k: fix Tx prepare to set positive rte_errno

Fixes: 9b134aa39716 ("net/fm10k: add Tx preparation")
Cc: stable@dpdk.org

Signed-off-by: Andrew Rybchenko <ac94ab2a8fe9a9a087f6e24dcbc16626b52c07e8@solarflare.com>
Acked-by: Xiao Wang <28aecb4e45c1566b6185c24680691da84bd4fcf8@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk