virtio: fix off by one device id comparison

VIRTIO_ID_MAX_NUM is the largest device ID plus 1. Therefore a device id
cannot be greater or equal to VIRTIO_ID_MAX_NUM. Fix the comparison
accordingly.

Fixes: 8fb49b4c ("dm: Add a new uclass driver for VirtIO transport devices")
Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>