fxas21002c:Clean and speed up with SW, HW LPF.

  Added paramater based low pass fileter settting from
  IMU_GYRO_CUTOFF.

  Added interfaces for setting the HW low pass filter.
  Set HW LPF to 64 Hz

  Changed poll rate to ODR to 800 Hz.

  Documeted the TIMER_REDUCTION value as 20% and changed it to
  250 Us / 1250 us.

  Added standby control API, to insure chip is configured in
  standby mode.

  removed tabs.

  removed gotos.