can: c_can: add a comment about IF_RX interface's use

After reading the commit 640916db

 ("can: c_can: Make it SMP safe")
it may sound strange to see the IF_RX interface used by the
can_inval_tx_object function. A comment was added to avoid any
misunderstanding.

Link: https://lore.kernel.org/r/20210302215435.18286-4-dariobin@libero.it
Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>