Preserve per-ASIC subdirectory structure for sdk_dbg collection

[william8545]

What I did

Fixed generate_dump to preserve per-ASIC subdirectory structure when collecting sdk_dbg files into techsupport dumps.

How I did it

Extract the relative subdirectory from each file's path under /var/log/sdk_dbg/ and use it as part of the dump destination (e.g., sai_sdk_dump/0/, sai_sdk_dump/1/), instead of saving all files flat into sai_sdk_dump/. This avoids basename collisions when multiple ASICs produce files with the same timestamp. Single-ASIC systems are unaffected.

How to verify it

1. Extract the dump and verify sai_sdk_dump/ contains per-ASIC subdirectories (0/, 1/, etc.) with each ASIC's pcap files preserved separately.
2. On a single-ASIC switch, verify sai_sdk_dump/ remains flat with no regressions.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[mssonicbld]

/azp run

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