bnxt_en: add HWRM request assignment API

hwrm_req_replace() provides an assignment like operation to replace a
managed HWRM request object with data from a pre-built source. This is
useful for handling request data provided by higher layer HWRM clients.

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>