usb: gadget: uvc: configfs: Don't wrap groups unnecessarily

Various configfs groups (represented by config_group) are wrapped in
structures that they're the only member of. This allows adding other
data fields to groups, but it unnecessarily makes the code more complex.
Remove the outer structures and use config_group directly to simplify
the code. Groups can still be wrapped individually in the future if
other data fields need to be added.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>