media: staging/imx: Fix NULL deref in find_pipeline_entity()

Fix a cut&paste error in find_pipeline_entity(). The start entity must be
passed to media_entity_to_video_device() in find_pipeline_entity(), not
pad->entity. The pad is only put to use later, after determining the start
entity is not the entity being searched for.

Fixes: 3ef46bc9 ("media: staging/imx: Improve pipeline searching")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>