sensor: adxl345: Fix decoder for non-streaming mode

The following fixes have been applied to this decoder:
- The Q-scale factor was fixed, both for full-scale and non
full-scale modes.
- The data-type decoded is struct sensor_three_axis_data, as
it should for read/decode API.

Signed-off-by: Luis Ubieda <luisf@croxel.com>