bnxt_en: Add a new BNXT_FW_RESET_STATE_POLL_FW_DOWN state.

This new state is required when firmware indicates that the error
recovery process requires polling for firmware state to be completely
down before initiating reset.  For example, firmware may take some
time to collect the crash dump before it is down and ready to be
reset.

Signed-off-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>