asus_acpi: fix possible double free (found by Coverity)

Signed-off-by: NJesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: NLen Brown <len.brown@intel.com>

drivers/acpi/asus_acpi.c

@@ -1192,6 +1192,7 @@ static int asus_hotk_get_info(void)
 			break;
 		default:
 			kfree(model);
+			model = NULL;
 			break;
 		}
 	}