bnxt_en: New encoding for the TX opaque field

In order to support multiple TX rings on the same MSIX, we'll use the
upper byte of the TX opaque field to store the ring index in the new
tx_napi_idx field.  This tx_napi_idx field is currently always 0 until
more infrastructure is added in later patches.

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