can: slcan: use the alloc_can_skb() helper

It is used successfully by most (if not all) CAN device drivers. It
allows to remove replicated code.

Link: https://lore.kernel.org/all/20220628163137.413025-4-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>