staging: media: ipu3: Drop superfluous check in imgu_vb2_stop_streaming()

The check for imgu_all_nodes_streaming() seems superfluous, since
imgu->streaming can only become true once imgu_all_nodes_streaming()
has been true. Hence, checking for imgu->streaming == true should
imply imgu_all_nodes_streaming(), and therefore suffice.

Signed-off-by: Max Staudt <mstaudt@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>