iio: st_sensors: drop temporary kmalloc buffer and reuse buffer_data

iio: st_sensors: drop temporary kmalloc buffer and reuse buffer_data

Replace the per-call kmalloc() scratch buffer with the existing
buffer_data[] field present in struct st_sensor_data. The existing buffer
is DMA-aligned and sufficiently sized for all channel widths, so using it
avoids unnecessary dynamic memory allocation on each read.

This simplifies the code, removes redundant allocation and cleanup.
No functional change intended.

Signed-off-by: Sanjay Chitroda <sanjayembeddedse@gmail.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>