pwm: lp3943: Fix duty calculation in case period was clamped

The hardware only supports periods <= 1.6 ms and if a bigger period is
requested it is clamped to 1.6 ms. In this case duty_cycle might be bigger
than 1.6 ms and then the duty cycle register is written with a value
bigger than LP3943_MAX_DUTY. So clamp duty_cycle accordingly.

Fixes: af66b3c0 ("pwm: Add LP3943 PWM driver")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>