net: can: at91_can: world-writable sysfs files

Don't allow everybody to write to mb0_id file.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
Acked-by: Kurt Van Dijck <kurt.van.dijck@eia.be>
Signed-off-by: David S. Miller <davem@davemloft.net>