soc: aspeed: lpc-snoop: Don't disable channels that aren't enabled

Mitigate e.g. the following:

# echo 1e789080.lpc-snoop > /sys/bus/platform/drivers/aspeed-lpc-snoop/unbind
...
[ 120.363594] Unable to handle kernel NULL pointer dereference at virtual address 00000004 when write
[ 120.373866] [00000004] *pgd=00000000
[ 120.377910] Internal error: Oops: 805 [#1] SMP ARM
[ 120.383306] CPU: 1 UID: 0 PID: 315 Comm: sh Not tainted 6.15.0-rc1-00009-g926217bc7d7d-dirty #20 NONE
...
[ 120.679543] Call trace:
[ 120.679559] misc_deregister from aspeed_lpc_snoop_remove+0x84/0xac
[ 120.692462] aspeed_lpc_snoop_remove from platform_remove+0x28/0x38
[ 120.700996] platform_remove from device_release_driver_internal+0x188/0x200
...

Fixes: 9f4f9ae81d0a ("drivers/misc: add Aspeed LPC snoop driver")
Cc: [e-mail]
Cc: Jean Delvare <[e-mail]>
Acked-by: Jean Delvare <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Andrew Jeffery <[e-mail]>

drivers/soc/aspeed/aspeed-lpc-snoop.c

···
 };
 
 struct aspeed_lpc_snoop_channel {
+	bool enabled;
 	struct kfifo		fifo;
 	wait_queue_head_t	wq;
 	struct miscdevice	miscdev;
···
 	const struct aspeed_lpc_snoop_model_data *model_data =
 		of_device_get_match_data(dev);
 
+	if (WARN_ON(lpc_snoop->chan[channel].enabled))
+		return -EBUSY;
+
 	init_waitqueue_head(&lpc_snoop->chan[channel].wq);
 	/* Create FIFO datastructure */
 	rc = kfifo_alloc(&lpc_snoop->chan[channel].fifo,
···
 		regmap_update_bits(lpc_snoop->regmap, HICRB,
 				hicrb_en, hicrb_en);
 
+	lpc_snoop->chan[channel].enabled = true;
+
 	return 0;
 
 err_misc_deregister:
···
 static void aspeed_lpc_disable_snoop(struct aspeed_lpc_snoop *lpc_snoop,
 				     int channel)
 {
+	if (!lpc_snoop->chan[channel].enabled)
+		return;
+
 	switch (channel) {
 	case 0:
 		regmap_update_bits(lpc_snoop->regmap, HICR5,
···
 		return;
 	}
 
+	lpc_snoop->chan[channel].enabled = false;
+	/* Consider improving safety wrt concurrent reader(s) */
 	misc_deregister(&lpc_snoop->chan[channel].miscdev);
 	kfifo_free(&lpc_snoop->chan[channel].fifo);
 }