igc: Reduce TSN TX packet buffer from 7KB to 5KB per queue

In the Linux kernel, the following vulnerability has been resolved:

igc: Reduce TSN TX packet buffer from 7KB to 5KB per queue

The previous 7 KB per queue caused TX unit hangs under heavy timestamping load. Reducing to 5 KB avoids these hangs and matches the TSN recommendation in I225/I226 SW User Manual Section 7.5.4.

The 8 KB "freed" by this change is currently unused. This reduction is not expected to impact throughput, as the i226 is PCIe-limited for small TSN packets rather than TX-buffer-limited.