bnxt_en: Allocate completion ring structures for 57500 series chips.

On 57500 chips, the original bnxt_cp_ring_info struct now refers to the
NQ.  bp->cp_nr_rings refer to the number of NQs on 57500 chips.  There
are now 2 pointers for the CP rings associated with RX and TX rings.
Modify bnxt_alloc_cp_rings() and bnxt_free_cp_rings() accordingly.

With multiple CP rings per NAPI, we need to add a pointer in
bnxt_cp_ring_info struct to point back to the bnxt_napi struct.

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