firmware: thead,th1520-aon: Fix use after free in th1520_aon_init()

Record the error code before freeing "aon_chan" to avoid a
use after free.

Fixes: e4b3cbd840e5 ("firmware: thead: Add AON firmware protocol driver")
Signed-off-by: Dan Carpenter <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ulf Hansson <[e-mail]>

drivers/firmware/thead,th1520-aon.c

···
 {
 	struct th1520_aon_chan *aon_chan;
 	struct mbox_client *cl;
+	int ret;
 
 	aon_chan = kzalloc(sizeof(*aon_chan), GFP_KERNEL);
 	if (!aon_chan)
···
 	aon_chan->ch = mbox_request_channel_byname(cl, "aon");
 	if (IS_ERR(aon_chan->ch)) {
 		dev_err(dev, "Failed to request aon mbox chan\n");
+		ret = PTR_ERR(aon_chan->ch);
 		kfree(aon_chan);
-		return ERR_CAST(aon_chan->ch);
+		return ERR_PTR(ret);
 	}
 
 	mutex_init(&aon_chan->transaction_lock);