bnxt_en: Restructure cp_ring_arr in struct bnxt_cp_ring_info

The cp_ring_arr is currently a fixed array of 2 pointers for the
TX and RX completion rings.  These pointers are allocated during
ring initialization.  Currntly, we support up to 2 completion rings
for each MSIX.  In order to support more completion rings, we change
this fixed array to a pointer and allocate the required entries
during ring initialization.  This patch keeps the current scheme of
allocating only 2 entries when needed.  Later patches will expand
and allocate more entries when required.

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>