can: raw: use temp variable instead of rolling back config

Introduce a temporary variable to check for an invalid configuration
attempt from user space. Before this patch the value was copied to
the real config variable and rolled back in the case of an error.

Suggested-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/all/20230203090807.97100-1-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>