drivers: sensor: adxl345: Correct converting counts to sensor value

Sensor value unit is [m/s^2].
Currently calcs as [g/1000] unit.

Correcting calculation to convert [m/s^2].
Store the integer part to val1, and the fractional part to val2.

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@fujitsu.com>