iio: kfifo: un-export devm_iio_kfifo_allocate() function

iio: kfifo: un-export devm_iio_kfifo_allocate() function

At this point all drivers should use devm_iio_kfifo_buffer_setup() instead
of manually allocating via devm_iio_kfifo_allocate() and assigning ops and
modes.

With this change, the devm_iio_kfifo_allocate() will be made private to the
IIO core, since all drivers should call either
devm_iio_kfifo_buffer_setup() or devm_iio_triggered_buffer_setup() to
create a kfifo buffer.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-6-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>