staging: comedi: complain if dma buffer allocation not supported

When allocating a buffer to support asynchronous comedi commands, if a
DMA coherent buffer was requested but `CONFIG_HAS_DMA` is undefined,
bail out of local helper function `__comedi_buf_alloc()` with an error
message.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_buf.c

@@ -76,6 +76,12 @@ static void __comedi_buf_alloc(struct comedi_device *dev,
 	struct comedi_buf_page *buf;
 	unsigned i;
 
+	if (!IS_ENABLED(CONFIG_HAS_DMA) && s->async_dma_dir != DMA_NONE) {
+		dev_err(dev->class_dev,
+			"dma buffer allocation not supported\n");
+		return;
+	}
+
 	async->buf_page_list = vzalloc(sizeof(*buf) * n_pages);
 	if (async->buf_page_list)
 		pages = vmalloc(sizeof(struct page *) * n_pages);