bnxt_en: Add a new BNXT_STATE_NAPI_DISABLED flag to keep track of NAPI state.

Up until now, we don't need to keep track of this state because NAPI
is always enabled once and disabled once during bring up and shutdown.
For better error recovery in subsequent patches, we want to quiesce
the device earlier during fatal error conditions.  The normal shutdown
sequence will disable NAPI again and the flag will prevent disabling
NAPI twice.

Reviewed-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
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>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>