pwm: imx: Don't print an error on -EPROBE_DEFER

When getting the peripheral clock fails with -EPROBE_DEFER the driver is
usually probed again later and emitting an error message is irritating.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>