can: make struct proto const

can_ioctl is the only reason for struct proto to be non-const.
script/check-patch.pl suggests struct proto be const.

Setting the reference to the common can_ioctl() in all CAN protocols directly
removes the need to make the struct proto writable in af_can.c

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