[Mellanox] Update buffer calculation for platform SN6600 (#3948)

- What I did
Starting from SN6600, Mellanox/Nvidia switches need update buffer calculation.
Update buffer calculation for SN6600 and later platforms.

- Why I did it
Need update new calculation for SN6600 and later platforms.

- How I verified it
Verify in SN6600 to see if buffer calculation is updated.
Other platform, buffer calculation is same as before.

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Signed-off-by: Jianyue Wu <[email protected]>

Author: jianyuewu