AP_InertialSensor: fixed detection of dead IMU

if a PX4 sensor does not give new data we need to avoid calling
_rotate_and_offset_*() to avoid marking it as healthy. Otherwise if
the MPU6k dies we won't switch to the LSM303D automatically
HAL_Linux: fixed Replay with new AP_InertialSensor library
Replay: cope with up to 3 IMUs