[media] video/saa7164: Fix sparse warning: Using plain integer as NULL pointer

This patch fixes the warning "Using plain integer as NULL pointer",
generated by sparse, by replacing
	if (var == 0)
with
	if (!var)
after an allocation
and all other offending 0s with NULL.

KernelVersion: linus' tree-1f0324c

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>