media: rkisp1: isp: Rename rkisp1_device.active_sensor to source

The active_sensor field of the rkisp1_device structure points to the ASD
data for the active source. The source may however not be a sensor, so
the naming is a bit confusing. Furthermore, the driver doesn't need to
access the full ASD from the active_sensor field, only the subdev
pointer is needed, when stopping streaming.

Rename the field to source, and turn it into a v4l2_subdev pointer.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Dafna Hirschfeld <dafna@fastmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>