bnxt_en: Set default completion ring for async events.

With the added support for the bnxt_re RDMA driver, both drivers can be
allocating completion rings in any order.  The firmware does not know
which completion ring should be receiving async events.  Add an
extra step to tell firmware the completion ring number for receiving
async events after bnxt_en allocates the completion rings.

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