If a valid power regulator or a dummy regulator is used (which
happens to be the case when no regulator is specified), restart_work
is queued no matter whether the device was running or not at suspend
time. Since work queues get initialized in the ndo_open callback,
resuming leads to a NULL pointer exception.

Reverse exactly the steps executed at suspend time:
- Enable the power regulator in any case
- Enable the transceiver regulator if the device was running, even in
  case we have a power regulator
- Queue restart_work only in case the device was running

Fixes: bf66f373 ("can: mcp251x: Move to threaded interrupts instead of workqueues.")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>