iio: imu: st_lsm6dsx: introduce conf_lock mutex

Add conf_lock mutex to prevent concurrent FIFO configuration update

Fixes: 290a6ce1 (iio: imu: add support to lsm6dsx driver)
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>