usb gadget: uvc: uvc_request_data::length field must be signed

commit 6f6543f53f9ce136e01d7114bf6f0818ca54fb41 upstream.

The field is used to pass the UVC request data length, but can also be
used to signal an error when setting it to a negative value. Switch from
unsigned int to __s32.

Reported-by: Fernandez Gonzalo <gfernandez@copreci.es>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>