bnxt_en: Support up to 8 TX rings per MSIX

For each mqprio TC, we allocate a set of TX rings to map to the new
hardware CoS queue.  Expand the tx_ring pointer in struct bnxt_napi
to an array of 8 to support up to 8 TX rings, one for each TC.
Only array entry 0 is used at this time.  The rest of the array
entries will be used 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>