In commit 20dd3850 "can: Speed up CAN frame
receiption by using ml_priv" the formerly used hlist of receiver lists for
each CAN netdevice has been replaced. 

The hlist content ensured only CAN netdevices to be accessed by the
can_rx_(un)register() functions which accidently dropped away together with
the hlist receiver implementation.

This patch re-introduces the check for CAN netdevices in can_rx_(un)register().
Signed-off-by: NOliver Hartkopp <oliver@hartkopp.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>