can: flexcan: disable runtime PM if register flexcandev failed

Disable runtime PM if register flexcandev failed, and balance reference
of usage_count.

Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Link: https://lore.kernel.org/r/20200929211557.14153-4-qiangqing.zhang@nxp.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>