KVM: arm: Use PTR_ERR_OR_ZERO()

Fix ptr_ret.cocci warnings:
virt/kvm/arm/vgic/vgic-its.c:971:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: Vasyl Gomonovych <gomonovych@gmail.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>