iio: mma8452: Add missed iio_device_unregister() call in mma8452_probe()

The function iio_device_register() was called in mma8452_probe().
But the function iio_device_unregister() was not called after
a call of the function mma8452_set_freefall_mode() failed.
Thus add the missed function call for one error case.

Fixes: 1a965d40 ("drivers:iio:accel:mma8452: added cleanup provision in case of failure.")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>