iio: trigger: stm32-timer: disable master mode when stopping

Master mode should be disabled when stopping. This mainly impacts
possible other use-case after timer has been stopped. Currently,
master mode remains set (from start routine).

Fixes: 6fb34812

 ("iio: stm32 trigger: Add support for TRGO2 triggers")

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>