can: make struct can_proto const

commit 53914b67

 had the
same message. That commit did put everything in place but
did not make can_proto const itself.

Signed-off-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>