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/sandbox-mbox.c b/drivers/mailbox/sandbox-mbox.c
index bc917b3..442ca63 100644
--- a/drivers/mailbox/sandbox-mbox.c
+++ b/drivers/mailbox/sandbox-mbox.c
@@ -87,7 +87,7 @@
 
 struct mbox_ops sandbox_mbox_mbox_ops = {
 	.request = sandbox_mbox_request,
-	.free = sandbox_mbox_free,
+	.rfree = sandbox_mbox_free,
 	.send = sandbox_mbox_send,
 	.recv = sandbox_mbox_recv,
 };