bnxt_en: Add completion ring pointer in TX and RX ring structures

From the TX or RX ring structure, we need to find the corresponding
completion ring during initialization.  On P5 chips, we use the MSIX/napi
entry to locate the completion ring because there is only one RX/TX
ring per MSIX.  To allow multiple TX rings for each MSIX, we need
to add a direct pointer from the TX ring and RX ring structures.
This also simplifies the existing logic.

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>