iio:iio_buffer_init(): Only set watermark if not already set

Only initialize the watermark field if it is still 0. This allows drivers
to provide a custom default watermark value. E.g. some driver might have a
fixed watermark or can only support watermarks within a certain range and
the initial value for the watermark should be within this range.

Signed-off-by: Lars-Peter Clausen <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/industrialio-buffer.c

···
 	INIT_LIST_HEAD(&buffer->buffer_list);
 	init_waitqueue_head(&buffer->pollq);
 	kref_init(&buffer->ref);
-	buffer->watermark = 1;
+	if (!buffer->watermark)
+		buffer->watermark = 1;
 }
 EXPORT_SYMBOL(iio_buffer_init);