bnxt_en: Add tx ring mapping logic.

To support XDP_TX, we need to add a set of dedicated TX rings, each
associated with the NAPI of an RX ring.  To assign XDP rings and regular
rings in a flexible way, we add a bp->tx_ring_map[] array to do the
remapping.  The netdev txq index is stored in the new field txq_index
so that we can retrieve the netdev txq when handling TX completions.
In this patch, before we introduce XDP_TX, the mapping is 1:1.

v2: Fixed a bug in bnxt_tx_int().

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>