usb: gadget: webcam: Make g_webcam loadable again

usb: gadget: webcam: Make g_webcam loadable again

commit 588b9e85609b ("usb: gadget: uvc: add v4l2 enumeration api calls")
has rendered the precomposed (aka legacy) webcam gadget unloadable.

uvc_alloc() since then has depended on certain config groups being
available in configfs tree related to the UVC function. However, legacy
gadgets do not create anything in configfs, so uvc_alloc() must fail
with -ENOENT no matter what.

This patch mimics the required configfs hierarchy to satisfy the code which
inspects formats and frames found in uvcg_streaming_header.

This has been tested with guvcview on the host side, using vivid as a
source of video stream on the device side and using the userspace program
found at https://gitlab.freedesktop.org/camera/uvc-gadget.git.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Fixes: 588b9e85609b ("usb: gadget: uvc: add v4l2 enumeration api calls")
Link: https://lore.kernel.org/r/20231215131614.29132-1-andrzej.p@collabora.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>