PCC: fix dereference of ERR_PTR

get_pcc_channel() does not return NULL on error it returns the error code
in ERR_PTR, but we have been checking it for NULL.
Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>

drivers/mailbox/pcc.c

@@ -122,7 +122,7 @@ struct mbox_chan *pcc_mbox_request_channel(struct mbox_client *cl,
 	 */
 	chan = get_pcc_channel(subspace_id);
 
-	if (!chan || chan->cl) {
+	if (IS_ERR(chan) || chan->cl) {
 		dev_err(dev, "Channel not found for idx: %d\n", subspace_id);
 		return ERR_PTR(-EBUSY);
 	}