can: af_can: give variable holding the CAN receiver and the receiver list a sensible name

This patch gives the variables holding the CAN receiver and the receiver
list a better name by renaming them from "r to "rcv" and "rl" to
"recv_list".

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>