can: slcan: use netdev helpers to print out messages

Replace printk() calls with corresponding netdev helpers.

Link: https://lore.kernel.org/all/20220628163137.413025-3-dario.binacchi@amarulasolutions.com
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Tested-by: Jeroen Hofstee <jhofstee@victronenergy.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>