iio: trigger: stm32-timer: fix get trigger mode

Fix reading trigger mode, when other bit-fields are set. SMCR register
value must be masked to read SMS (slave mode selection) only.

Fixes: 9eba381b ("iio: make stm32 trigger driver use
INDIO_HARDWARE_TRIGGERED mode")

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