vdev_id: Fix PHY sorting

One of our developers noticed a bug in vdev_id where we were incorrectly
sorting PHYs using alphabetical sorting (which usually works) instead
of natural sorting (-v). For example:

[port-0:0]# ls -d phy*
phy-0:10 phy-0:11 phy-0:8 phy-0:9

[port-0:0]# ls -vd phy*
phy-0:8 phy-0:9 phy-0:10 phy-0:11

This fixes the issue.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #12699