pwm: bcm2835: Suppress error message for invalid period_ns

The PWM config can be triggered via sysfs, so we better suppress the
error message in case of an invalid period to avoid kernel log spamming.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>