iio: buffer: check return value of iio_compute_scan_bytes()

iio: buffer: check return value of iio_compute_scan_bytes()

Check return value of iio_compute_scan_bytes() as it can return an
error.

The result is moved to an output parameter while we are touching this
as we will need to add a second output parameter in a later change.

The return type of iio_buffer_update_bytes_per_datum() also had to be
changed to propagate the error.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>