mailbox: check ops prior calling

Check if request and free operations are present prior calling to the
functions.

Signed-off-by: Ibai Erkiaga <ibai.erkiaga-elorza@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/drivers/mailbox/mailbox-uclass.c b/drivers/mailbox/mailbox-uclass.c
index 1b4a586..809f26b 100644
--- a/drivers/mailbox/mailbox-uclass.c
+++ b/drivers/mailbox/mailbox-uclass.c
@@ -63,7 +63,8 @@
 		return ret;
 	}
 
-	ret = ops->request(chan);
+	if (ops->request)
+		ret = ops->request(chan);
 	if (ret) {
 		debug("ops->request() failed: %d\n", ret);
 		return ret;
@@ -94,7 +95,10 @@
 
 	debug("%s(chan=%p)\n", __func__, chan);
 
-	return ops->free(chan);
+	if (ops->free)
+		return ops->free(chan);
+
+	return 0;
 }
 
 int mbox_send(struct mbox_chan *chan, const void *data)