bnxt_en: remove DMA mapping for KONG response

The additional response buffer serves no useful purpose. There can
be only one firmware command in flight due to the hwrm_cmd_lock mutex,
which is taken for the entire duration of any command completion,
KONG or otherwise. It is thus safe to share a single DMA buffer.

Removing the code associated with the additional mapping will simplify
matters in the next patch, which allocates response buffers from DMA
pools on a per request basis.

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