[media] smiapp: Set left and top to zero for crop bounds selection

The fields were previously uninitialised, leaving the returned values to
where the user had set them. This was never the intention.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>