can: slcan: use the generic can_change_mtu()

It is useless to define a custom function that does nothing but always
return the same error code. Better to use the generic can_change_mtu()
function.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Link: https://lore.kernel.org/all/20220728070254.267974-6-dario.binacchi@amarulasolutions.com


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>