bnxt_en: Improve ring allocation logic.

bnxt_en: Improve ring allocation logic.

Currently, the driver code makes some assumptions about the group index
and the map index of rings.  This makes the code more difficult to
understand and less flexible.

Improve it by adding the grp_idx and map_idx fields explicitly to the
bnxt_ring_struct as a union.  The grp_idx is initialized for each tx ring
and rx agg ring during init. time.  We do the same for the map_idx for
each cmpl ring.

The grp_idx ties the tx ring to the ring group.  The map_idx is the
doorbell index of the ring.  With this new infrastructure, we can change
the ring index mapping scheme easily in the future.

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