MAINTAINER: Correct CAN Maintainer responsibilities and paths

Update the CAN Maintainer responsibilities and add source paths.
Additional the SocketCAN core ML is not subscribers-only anymore.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Wolfgang Grandegger <wg@grandegger.com>
Signed-off-by: David S. Miller <davem@davemloft.net>