usb: gadget: uvc: Set the vb2 queue timestamp flags

The vb2 queue timestamp_flags field must be set by drivers, as enforced
by a WARN_ON in vb2_queue_init. The UVC gadget driver failed to do so.
This resulted in the following warning.

[    2.104371] g_webcam gadget: uvc_function_bind
[    2.105567] ------------[ cut here ]------------
[    2.105567] ------------[ cut here ]------------
[    2.106779] WARNING: CPU: 0 PID: 1 at drivers/media/v4l2-core/videobuf2-core.c:2207 vb2_queue_init+0xa3/0x113()

Fix it.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>