[media] omap3isp: Remove unneeded memset after kzalloc

kzalloc initializes the memory it allocates to 0, there's no need for an
explicit memset.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>