staging:iio: Disallow changing scan elements in all buffered modes

Currently we only disallow changing the scan elements, while the buffer is
enabled, in triggered buffer mode. This patch changes it to disallow it for all
buffered modes. Disabling or enabling scan elements while the buffer is enabled
will cause undefined behavior since the reader will not be able to tell samples
with the new and old scan element set apart and thus wont be able to extract
any meaningful data from the buffer.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>