fw_cfg: Constify struct kobj_type

This 'struct kobj_type' is not modified. It is only used in
kobject_init_and_add() which takes a 'const struct kobj_type *ktype'
parameter.

Constifying this structure and moving it to a read-only section,
and this can increase over all security.

```
[Before]
   text   data    bss    dec    hex    filename
   5974   1008     96   7078   1ba6    drivers/firmware/qemu_fw_cfg.o

[After]
   text   data    bss    dec    hex    filename
   6038    944     96   7078   1ba6    drivers/firmware/qemu_fw_cfg.o
```
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Message-Id: <20240904011743.2010319b-1-lihongbo22@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/firmware/qemu_fw_cfg.c

@@ -452,7 +452,7 @@ static void fw_cfg_sysfs_release_entry(struct kobject *kobj)
 }
 
 /* kobj_type: ties together all properties required to register an entry */
-static struct kobj_type fw_cfg_sysfs_entry_ktype = {
+static const struct kobj_type fw_cfg_sysfs_entry_ktype = {
 	.default_groups = fw_cfg_sysfs_entry_groups,
 	.sysfs_ops = &fw_cfg_sysfs_attr_ops,
 	.release = fw_cfg_sysfs_release_entry,