virtio-mmio: Use PTR_ERR_OR_ZERO()

Fix ptr_ret.cocci warnings:
drivers/virtio/virtio_mmio.c:653:1-3: 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: Michael S. Tsirkin <mst@redhat.com>