xhci: dbgtty: Fix unregister on tty_register_driver() failure

If tty_register_driver() fails, it drops the reference, but fails to set
the global dbc_tty_driver to NULL, causing the unregister to be called
again when module exits.

On module unload dbc_tty_exit() only gates its cleanup on the driver
pointer being non-NULL, so it operates on the already-freed driver:

    module_init(xhci_hcd_init)
      xhci_hcd_init()
        xhci_dbc_init()                       [return value ignored]
          dbc_tty_init()
            tty_register_driver() fails
              tty_driver_kref_put()           -> driver freed
              (dbc_tty_driver left dangling)
    ...
    module_exit(xhci_hcd_fini)
      xhci_hcd_fini()
        xhci_dbc_exit()
          dbc_tty_exit()
            if (dbc_tty_driver)               -> true (dangling)
              tty_unregister_driver()         -> use-after-free

Fixes: 4521f16139 ("xhci: dbctty: split dbc tty driver registration and unregistration functions.")
Cc: stable@vger.kernel.org # v5.10
Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lucas De Marchi <ldemarchi@nvidia.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Link: https://patch.msgid.link/20260806142113.2436238-8-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-dbgtty.c

@@ -651,6 +651,7 @@ int dbc_tty_init(void)
 		pr_err("Can't register dbc tty driver\n");
 		tty_driver_kref_put(dbc_tty_driver);
 		idr_destroy(&dbc_tty_minors);
+		dbc_tty_driver = NULL;
 	}
 
 	return ret;