firmware: turris-mox-rwtm: Do not complete if there are no waiters

Do not complete the "command done" completion if there are no waiters.
This can happen if a wait_for_completion() timed out or was interrupted.

Fixes: 389711b ("firmware: Add Turris Mox rWTM firmware driver")
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Author: elkablo

drivers/firmware/turris-mox-rwtm.c

@@ -2,7 +2,7 @@
 /*
  * Turris Mox rWTM firmware driver
  *
- * Copyright (C) 2019 Marek Behún <kabel@kernel.org>
+ * Copyright (C) 2019, 2024 Marek Behún <kabel@kernel.org>
  */
 
 #include <linux/armada-37xx-rwtm-mailbox.h>
@@ -174,6 +174,9 @@ static void mox_rwtm_rx_callback(struct mbox_client *cl, void *data)
 	struct mox_rwtm *rwtm = dev_get_drvdata(cl->dev);
 	struct armada_37xx_rwtm_rx_msg *msg = data;
 
+	if (completion_done(&rwtm->cmd_done))
+		return;
+
 	rwtm->reply = *msg;
 	complete(&rwtm->cmd_done);
 }