iio: Un-inline iio_buffer_enabled()

iio: Un-inline iio_buffer_enabled()

As we are going to hide the currentmode inside the opaque structure,
this helper would soon need to call a non-inline function which would
simply drop the benefit of having the helper defined inline in a header.

One alternative is to move this helper in the core as there is no more
interest in defining it inline in a header. We will pay the minor cost
either way.

Let's do like the iio_device_id() helper which also refers to the opaque
structure and gets defined in the core.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-10-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>