can: peak_pci: prevent use after free at netdev removal

As remarked by Christopher R. Baker in his post at

http://marc.info/?l=linux-can&m=139707295706465&w=2



there's a possibility for an use after free condition at device removal.

This simplified patch introduces an additional variable to prevent the issue.
Thanks for catching this.

Cc: linux-stable <stable@vger.kernel.org>
Reported-by: Christopher R. Baker <cbaker@rec.ri.cmu.edu>
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>