net/vnstat: use OPNsense interface names in dashboard widget

[joeroback]

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Describe the bug
https://forum.opnsense.org/index.php?topic=51328.0

User asked to use the OPNsense interface names in the vnstat dashboard widget dropdown instead of the FreeBSD interface names.

To Reproduce
Add vnstat widget to dashboard

Expected behavior
The dashboard widget should use the OPNsense interface names, not the FreeBSD interface names

Screenshots

Relevant log files

Additional context

Environment