[media] v4l: atmel-isi: Simplify error handling during DT parsing

Put the endpoint DT node earlier to avoid the need for goto statements
to a cleanup code block in case of errors.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>