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: Robin Schroer <[e-mail]>
Acked-by: Alan Cox <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/staging/goldfish/goldfish_audio.c

···
 	}
 	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;
 	}