media: ipu6: Obtain unique source pad from remote sub-device

media: ipu6: Obtain unique source pad from remote sub-device

Obtain unique source pad from a remote sub-device, instead of the first
one. This means that only one link may be active at stream start. There's
no functional change in practice, unless multiple CSI-2 transmitters are
directly connected to the receiver.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Bingbu Cao <bingbu.cao@intel.com>
Tested-by: Mehdi Djait <mehdi.djait@linux.intel.com> # Dell XPS 9315
Reviewed-by: Mehdi Djait <mehdi.djait@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>