media: intel/ipu6: Fix a null pointer dereference in ipu6_isys_query_stream_by_source

stream is NULL when source is less than 0 in
ipu6_isys_query_stream_by_source. It's a null pointer dereference.

Actually, this should be isys->adev->auxdev.dev.

Fixes: 3c1dfb5a

 ("media: intel/ipu6: input system video nodes and buffer queues")
Signed-off-by: Hao Ge <gehao@kylinos.cn>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>