iio: trigger: stm32-timer: fix write_raw return value

Fixes: 4adec7da ("iio: stm32 trigger: Add quadrature encoder device")

IIO core expects zero as return value for write_raw() callback
in case of success.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Acked-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>