MAINTAINERS: The socketcan-core list is subscribers-only.

When I posted a copy_to_user fixes, the list daemon refused to accept
the Cc: <this-list>, because I was not a subscriber.

I found, that other lists with such a feature are marked respectively
in the MAINTAINERS file.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Oliver Hartkopp <oliver.hartkopp@volkswagen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>