net/bnxt: optimize Tx batching

This patch adds a few optimizations to Tx batching. Instead of
specifying 'cmpl_next' to bnxt_start_xmit() as a hint to enable
completion for a given packet, request for completion on the last
successfully enqueued TxBD. This takes care of both success and
error cases. It eliminates 'cmpl_next' arg and related per-packet
condition checks around it.

Fixes: 5735eb241947 ("net/bnxt: support Tx batching")
Cc: stable@dpdk.org
Signed-off-by: Sriharsha Basavapatna <sriharsha.basavapatna@broadcom.com>
Signed-off-by: Ajit Khaparde <ajit.khaparde@broadcom.com>