media: staging/imx: fix two NULL vs IS_ERR() bugs

The imx_media_pipeline_pad() function return NULL pointers on error, it
never returns error pointers.

Fixes: 3ef46bc9

 ("media: staging/imx: Improve pipeline searching")

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>