usb: gadget: dfu: correct size for USB_REQ_DFU_GETSTATE result

return the correct size for DFU_GETSTATE result (1 byte in DFU 1.1 spec)
to avoid issue in USB protocol and the variable "value" is propagated
to req->lenght as all the in the other request with answer
- DFU_GETSTATUS
- DFU_DNLOAD
- DFU_UPLOAD
Then the buffer is correctly treated in USB driver

NB: it was the only request witch directly change "req->actual"

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay73@gmail.com>