media: staging: media: omap4iss: Replace NULL comparison.

This patch modifies NULL comparison to fix checkpatch.pl warning.

Signed-off-by: Nachammai Karuppiah <nachukannan@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>