net/thunderx: avoid sq door bell write on zero packet
net/thunderx: avoid sq door bell write on zero packet

Avoid sq door bell write on zero packet case to reduce additional
traffic on register bus.

Fixes: 1c421f18e0 ("net/thunderx: add single and multi-segment Tx")
Cc: stable@dpdk.org

Signed-off-by: Kiran Kumar <48af8290976b58a4b1612d6595def915135b1890@caviumnetworks.com>
Acked-by: Jerin Jacob <5bca7d2a432bc0f5422b70b2701a4e4fa32edd00@caviumnetworks.com>

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