pwm: atmel: Fix duty cycle calculation in .get_state()

The CDTY register contains the number of inactive cycles. .apply() does
this correctly, however .get_state() got this wrong.

Fixes: 651b510a ("pwm: atmel: Implement .get_state()")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>