virtio_blk: use UINT_MAX instead of -1U

We use UINT_MAX to limit max_discard_sectors in virtblk_probe,
we can use UINT_MAX to limit max_hw_sectors for consistencies.

No functional change intended.

Signed-off-by: Angus Chen <angus.chen@jaguarmicro.com>
Message-Id: <20221110030124.1986-1-angus.chen@jaguarmicro.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>