iio: kfifo: add devm_iio_kfifo_buffer_setup() helper

iio: kfifo: add devm_iio_kfifo_buffer_setup() helper

This change adds the devm_iio_kfifo_buffer_setup() helper/short-hand,
which groups the simple routine of allocating a kfifo buffers via
devm_iio_kfifo_allocate() and calling iio_device_attach_buffer().

The mode_flags parameter is required, as the IIO kfifo supports 2 modes:
INDIO_BUFFER_SOFTWARE & INDIO_BUFFER_TRIGGERED.
The setup_ops parameter is optional.

This function will be a bit more useful when needing to define multiple
buffers per IIO device.

The naming for this function has been inspired from
iio_triggered_buffer_setup() since that one does a kfifo alloc + a pollfunc
alloc. So, this should have a more familiar ring to what it is.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210215104043.91251-3-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>