iio: accel: adxl372: Fix push to buffers lost samples

commit 62df81b7

 upstream.

One in two sample sets was lost by multiplying fifo_set_size with
sizeof(u16). Also, the double number of available samples were pushed to
the iio buffers.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Fixes: f4f55ce3

 ("iio:adxl372: Add FIFO and interrupts support")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>