iio: adc: ad7887: use iio_push_to_buffers_with_ts()

Replace the deprecated iio_push_to_buffers_with_timestamp() with
iio_push_to_buffers_with_ts(), passing the size of the pushed buffer,
which is a structure within the iio_priv() data.

Replace the data buffer with one declared via
IIO_DECLARE_DMA_BUFFER_WITH_TS(), for 2 __be16 channels.

Use be16_to_cpu() to replace the opencoded equivalent.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jonathan.cameron@oss.qualcomm.com>