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.
2010319-1-lihongbo22@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>