usb: gadget: uvc: Make bSourceID read/write

usb: gadget: uvc: Make bSourceID read/write

[ Upstream commit b3c839bd8a07d303bc59a900d55dd35c7826562c ]

At the moment, the UVC function graph is hardcoded IT -> PU -> OT.
To add XU support we need the ability to insert the XU descriptors
into the chain. To facilitate that, make the output terminal's
bSourceID attribute writeable so that we can configure its source.

Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-2-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>