pch_can: Optimize "if" condition in rx/tx processing

For reduce "if" condition, easy to read/understand the code,
optimize "if" condition in rx/tx processing.

Signed-off-by: Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>