iio: accel: adxl372: Set iio_chan BE

Data stored in the iio-buffer is BE and this
should be specified in the iio_chan_spec struct.

Fixes: f4f55ce3 ("iio:adxl372: Add FIFO and interrupts support")
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>