usb: gadget: uvc: Merge the SS/HS/FS control endpoint descriptors

The descriptors are identical, there's no need to have several copies of
them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Bhupesh Sharma <bhupesh.sharma@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>