pwm: clps71xx: Implement .apply() callback

To eventually get rid of all legacy drivers convert this driver to the
modern world implementing .apply().

This fixes a small issue in clps711x_get_duty() en passant: the
multiplication v * 0xf might have overflown.

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