can: flexcan: fix 'passing zero to ERR_PTR()' warning

Fix a static code checker warning:
drivers/net/can/flexcan.c:1435 flexcan_setup_stop_mode() warn: passing zero to 'PTR_ERR'

Fixes: de3578c1

 ("can: flexcan: add self wakeup support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>