media: imx: csi: Allow unknown nearest upstream entities

On i.MX6, the nearest upstream entity to the CSI can only be the
CSI video muxes or the Synopsys DW MIPI CSI-2 receiver.

However the i.MX53 has no CSI video muxes or a MIPI CSI-2 receiver.
So allow for the nearest upstream entity to the CSI to be something
other than those.

Fixes: bf3cfaa7 ("media: staging/imx: get CSI bus type from nearest
upstream entity")

Signed-off-by: Steve Longerbeam <slongerbeam@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>