pwm: Replace ENOTSUPP with EOPNOTSUPP

According to Documentation/dev-tools/checkpatch.rst ENOTSUPP is
not recommended and EOPNOTSUPP should be used instead.

Signed-off-by: Sean Young <sean@mess.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>