bnxt_en: refactor reset close code

Introduce bnxt_fw_fatal_close() API which can be used
to stop data path and disable device when firmware
is in fatal state.

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