media: imx7: csi: Fix pad link validation

We can not make the assumption that the bound subdev is always a CSI
mux, in i.MX6UL/i.MX6ULL that is not the case. So, just get the entity
selected by source directly upstream from the CSI.

Fixes: 86e02d07

 ("media: imx5/6/7: csi: Mark a bound video mux as a CSI mux")
Reported-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>