TX timestamps were added to the can_put_echo_skb() function of can_dev
modules in [1]. However, janz-ican3 does not rely on that function but
instead implements its own echo_skb logic. As such it does not offer
TX timestamping.
Add a call to skb_tx_timestamp() in the ican3_put_echo_skb() function
so that the module now supports TX software timestamps.