media: atmel-isc: Remove unnecessary member

Remove the memeber *config from the isc_subdev_entity struct,
the member is useless afterward.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>