iio: accel: mma8452: use the correct logic to get mma8452_data

In the Linux kernel, the following vulnerability has been resolved:

iio: accel: mma8452: use the correct logic to get mma8452_data

The original logic to get mma8452_data is wrong, the *dev point to
the device belong to iio_dev. we can't use this dev to find the
correct i2c_client. The original logic happen to work because it
finally use dev->driver_data to get iio_dev. Here use the API
to_i2c_client() is wrong and make reader confuse. To correct the
logic, it should be like this

struct mma8452_data *data = iio_priv(dev_get_drvdata(dev));

But after commit 8b7651f25962 ("iio: iio_device_alloc(): Remove
unnecessary self drvdata"), the upper logic also can't work.
When try to show the avialable scale in userspace, will meet kernel
dump, kernel handle NULL pointer dereference.

So use dev_to_iio_dev() to correct the logic.

Dual fixes tags as the second reflects when the bug was exposed, whilst
the first reflects when the original bug was introduced.