mailbox: Rename free() to rfree()

This function name conflicts with our desire to #define free() to
something else on sandbox. Since it deals with resources, rename it to
rfree().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mailbox/mailbox-uclass.c b/drivers/mailbox/mailbox-uclass.c
index 5968c9b..a6d2d1f 100644
--- a/drivers/mailbox/mailbox-uclass.c
+++ b/drivers/mailbox/mailbox-uclass.c
@@ -105,8 +105,8 @@
 
 	debug("%s(chan=%p)\n", __func__, chan);
 
-	if (ops->free)
-		return ops->free(chan);
+	if (ops->rfree)
+		return ops->rfree(chan);
 
 	return 0;
 }