iio: imu: st_lsm6dsx: grab conf mutex in st_lsm6dsx_write_event_config

Always grub conf mutex in st_lsm6dsx_write_event_config since it can run
concurrently with FIFO configuration

Fixes: b5969abf ("iio: imu: st_lsm6dsx: add motion events")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>