can: slcan: move driver into separate sub directory

This patch moves the slcan driver into a separate directory, a later
patch will add more files.

Link: https://lore.kernel.org/all/20220628163137.413025-10-dario.binacchi@amarulasolutions.com
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>