net/bnxt: fix Tx descriptor status implementation

net/bnxt: fix Tx descriptor status implementation

With Tx completion batching, a single transmit completion
can correspond to one or more transmit descriptors, adjust
implementation to account for this.

RTE_ETH_TX_DESC_DONE should be returned for descriptors that
are available for use instead of RTE_ETH_TX_DESC_UNAVAIL.

Fixes: 5735eb241947 ("net/bnxt: support Tx batching")
Fixes: 478ed3bb7b9d ("net/bnxt: support Tx descriptor status")
Cc: stable@dpdk.org
Signed-off-by: Lance Richardson <lance.richardson@broadcom.com>
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>