sensors: use SENSOR_G constant in all accelerometer drivers

Use the SENSOR_G constant from sensor.h in all accelerometer
drivers, instead of having the constant redefined in each
driver header file.

Since GRAVITY_CONST was measured in mili-m/s^2 and SENSOR_G
is measured in micro-m/s^2, some changes to the calculation
of the acceleration data were added.

Change-Id: Ia323dfc46bee00035e24b37e1b7fbc886dfbe029
Signed-off-by: Bogdan Davidoaia <bogdan.m.davidoaia@intel.com>