serial: T8211: fix symlink collisions for multi-port USB serial adapters

[srnoth]

Change summary

Multi-port USB serial adapters where all ports share the same USB
ID_PATH (e.g., MosChip MCS7840) create identical symlink names in
/dev/serial/by-bus/, so only the last-enumerated port is accessible.

The existing udev rule captures .ID_PORT but never includes it in the
symlink name. This splits the catch-all .ID_PORT rule into two cases:

• Port 0: keeps the base symlink name (backward compatible)
• Port 1+: appends pN suffix following the existing naming scheme

Adapters where each port is a separate USB interface with a unique
ID_PATH (e.g., FTDI FT4232H) and single-port devices are unaffected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T8211

Related PR(s)

N/A

How to test / Smoketest result

Tested on VyOS 2026.02 (circinus) with:

• MosChip MCS7840 8-port adapter (2× 4-port chips)
• FTDI FT4232H 4-port adapter
• Single-port USB serial adapters

Before (MCS7840 — only 1 symlink per 4-port chip):

usb0b1.4.1p1.0 -> ../../ttyUSB7
usb0b1.4.2p1.0 -> ../../ttyUSB11

After (all 8 ports visible):

usb0b1.4.1p1.0   -> ../../ttyUSB6
usb0b1.4.1p1.0p1 -> ../../ttyUSB7
usb0b1.4.1p1.0p2 -> ../../ttyUSB8
usb0b1.4.1p1.0p3 -> ../../ttyUSB9
usb0b1.4.2p1.0   -> ../../ttyUSB11
usb0b1.4.2p1.0p1 -> ../../ttyUSB12
usb0b1.4.2p1.0p2 -> ../../ttyUSB13
usb0b1.4.2p1.0p3 -> ../../ttyUSB14

FTDI FT4232H and single-port devices unchanged. No applicable smoketest exists for udev rules.

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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[srnoth]

I have read the CLA Document and I hereby sign the CLA

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