media: ti-vpe: cal: rename "sensor" to "source"

CAL driver uses "sensor" name to refer to the subdev connected to CAL.
As the subdev can also be a bridge, the naming is misleading and might
cause the reader to think it refers to the actual sensor at the end of
the pipeline.

Rename "sensor" to "source".

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>