fw_cfg: fix sparse warning reading FW_CFG_ID

Use a restricted type for reading FW_CFG_ID, fixing sparse warning:

drivers/firmware/qemu_fw_cfg.c:540:22: warning: cast to restricted __le32

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>