[media] staging: media: cxd2099: fix sparse warnings in cxd2099_attach

The following sparse warnings were fixed

drivers/staging/media/cxd2099/cxd2099.c:686:26: warning: Using plain integer as NULL pointer
drivers/staging/media/cxd2099/cxd2099.c:691:24: warning: Using plain integer as NULL pointer
drivers/staging/media/cxd2099/cxd2099.c:696:24: warning: Using plain integer as NULL pointer

Signed-off-by: Devendra Naga <develkernel412222@gmail.com>
Cc: Ralph Metzler <rjkm@metzlerbros.de>
Cc: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>