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/stm32-ipcc.c b/drivers/mailbox/stm32-ipcc.c
index c3df967..d4035a8 100644
--- a/drivers/mailbox/stm32-ipcc.c
+++ b/drivers/mailbox/stm32-ipcc.c
@@ -152,7 +152,7 @@
 
 struct mbox_ops stm32_ipcc_mbox_ops = {
 	.request = stm32_ipcc_request,
-	.free = stm32_ipcc_free,
+	.rfree = stm32_ipcc_free,
 	.send = stm32_ipcc_send,
 	.recv = stm32_ipcc_recv,
 };