drivers: sensor: Fix adxl345 sample fetch return value

Fixes the adxl345 accelerometer driver to return zero on sample fetch
success, as specified by the sensor driver API. Prior to this change,
the driver incorrectly returned a positive non-zero value on sample
fetch success, which in turn caused a bus fault getting data with the
sensor shell.

Signed-off-by: Maureen Helm <maureen.helm@analog.com>