staging/goldfish/goldfish_audio: fix a sparse warning

Fix a pointer check to use NULL instead of 0

Warning:
drivers/staging/goldfish/goldfish_audio.c:300:34: warning: Using plain
integer as NULL pointer
Signed-off-by: NRobin Schroer <sulamiification@gmail.com>
Acked-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/goldfish/goldfish_audio.c

@@ -297,7 +297,7 @@ static int goldfish_audio_probe(struct platform_device *pdev)
 	}
 	data->buffer_virt = dmam_alloc_coherent(&pdev->dev,
 				COMBINED_BUFFER_SIZE, &buf_addr, GFP_KERNEL);
-	if (data->buffer_virt == 0) {
+	if (data->buffer_virt == NULL) {
 		dev_err(&pdev->dev, "allocate buffer failed\n");
 		return -ENOMEM;
 	}