checks: Use proper format modifier for size_t

The size of size_t can vary between architectures, so using %ld isn't
going to work on 32-bit builds. Use the %zu modifier to make sure it is
always correct.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>