usb: gadget: uvc: Fix string descriptor STALL issue when multiple uvc functions are...

This patch solved the string descriptor STALL issue when we add multiple UVC
functions in a single configuration using a 'webcam.c' like composite driver.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>