usb: gadget: uvc: Use custom strings if available

usb: gadget: uvc: Use custom strings if available

If the user has defined a custom string descriptor for the IAD or the
VideoStreaming interfaces then set their index field to point to the
custom descriptor instead of the hardcoded defaults. If no custom
descriptors have been linked to, then use the default ones.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-12-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>