media: sun6i-isp: params: Unregister pending buffer on cleanup

The state cleanup helper should unregister the pending buffer from
the state after returning it to v4l2, like it is done for other
buffers in the wait queue.

Before this change, the pending buffer from a previous run might have
been returned at the beginning of the next run, causing an error.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Fixes: e3185e1d ("media: staging: media: Add support for the Allwinner A31 ISP")
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>