staging: iio: ad5933: attach life-cycle of kfifo buffer to parent device and use managed calls throughout

This change makes the use of devm_iio_kfifo_allocate() to attach the
life-cycle of the kfifo buffer to the parent (client->dev) object.

This removes the need to explicitly free 'indio_dev->buffer' via
iio_kfifo_free(), which is the main intent.

Having done this, it is straight forward to move to devm_ calls throughout
and drop the remove function.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>