bnxt_en: Reply to firmware's echo request async message.

This is a new async message that the firmware can send to check if it
can communicate with the driver.  This is an added error detection
scheme that firmware can use if it suspects errors in the PCIe
interface.  When the driver receives this async message, it will reply
back echoing some data in the async message.  If the firmware is not
getting the reply with the proper data after some retries, error
recovery will kick in.

Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Reviewed-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>