media: rcar-vin: Rename rectangle holding the video source information

The variable to hold the video source information dimensions was poorly
named 'source'. This is confusing as a lot of other members of structs
share the same name with different purposes, rename it src_rect in
preparation of refactoring code.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@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+samsung@kernel.org>