pwm: ntxec: Use device_set_of_node_from_dev()

Compared to overwriting pdev->dev.of_node directly, this takes care of
reference counting. It also prevents that the parent device matches this
driver. See commit 9b22c17a ("of: Check 'of_node_reused' flag on
of_match_device()") for further details.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>