iio: iio_device_alloc(): Remove unnecessary self drvdata

Drvdata is typically used by drivers to attach driver specific data to a
device. It is used to retrieve driver specific information when only the
device to which the data is attached is available.

In the IIO core in the `iio_device_alloc()` function we call
`iio_device_set_drvdata(indio_dev, indio_dev)`. This sets the drvdata of
the IIO device to itself.

This is rather unnecessary since if we have a pointer to the IIO device to
call `iio_device_get_drvdata()` on it we don't need to call the function
since we already have the pointer. If we only have a pointer to the `struct
device` we can use `dev_to_iio_dev()` to get the IIO device from it.

Furthermore the drvdata is supposed to be reserved for drivers, so it
should not be used by the IIO core in the first place.

The `set_drvdata()` has been around from the very beginning of the IIO
framework and back then it was used in the IIO device sysfs attribute
handling code. But that was subsequently replaced with a `dev_to_iio_dev()`
in commit e53f5ac5

 ("iio: Use dev_to_iio_dev()") and other cleanups.

The self `set_drvdata()` is now no longer needed and can be removed.

Verified that there no longer any users by checking for potential users
using the following two coccinelle scripts and reviewing that none of the
matches are problematic code.

<smpl>
@@
struct iio_dev *iio_dev;
expression dev;
identifier fn !~ "(remove|resume|suspend)";
@@
fn(...)
{
...
*iio_dev = dev_get_drvdata(dev)
...
}
</smpl>

<smpl>
@r1@
position p;
struct iio_dev *indio_dev;
identifier dev_fn =~ "^dev_";
identifier devm_fn =~ "^devm_";
@@
(
 dev_fn
|
 devm_fn
)
 (&indio_dev@p->dev, ...)

@@
struct iio_dev *indio_dev;
position p != r1.p;
@@
*&indio_dev@p->dev</smpl>

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>