can: flexcan: fix an use-after-free in flexcan_setup_stop_mode()

The gpr_np variable is still being used in dev_dbg() after the
of_node_put() call, which may result in use-after-free.

Fixes: de3578c1 ("can: flexcan: add self wakeup support")
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Cc: linux-stable <stable@vger.kernel.org> # >= v5.0
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>