can: mscan: Fix Kconfig dependencies

The only driver based on MSCAN at the moment is for PPC machines,
so it makes no sense to present the menu on M68K. The menu will always
be empty there.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>