[media] atomisp: use NULL instead of 0 for pointers

Fix warning issued by sparse: Using plain integer as NULL pointer

Signed-off-by: Paolo Cretaro <melko@frugalware.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>