iwlwifi: mvm: avoid dumping assert log when device is stopped

We might erroneously get to error dumping code when the
device is already stopped.

In that case the driver will detect a defective value and will try to
reset the HW, assuming it is only a bus issue.  The driver than
proceeds with the dumping.

The result has two side effects:

1. The device won't be stopped again, since the transport status is
already stopped, so the device remains powered on while it actually
should be stopped.

2. The dump in that case is completely garbaged and useless.

Detect and avoid this.  It will also make debugging such issues
easier.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>