PCI_MSI_2 skips MSI-only endpoints in Linux vector uniqueness test

[chetan-rathore]

The PCI_MSI_2 rule applies to MSI(-X), so endpoints that support MSI only should be valid targets for the test.
In the current Linux ACS implementation, the vector uniqueness test appears to skip MSI-only endpoints because the capability check requires both MSI-X and MSI capability discovery to pass before the device is considered.
Current behavior:

• Endpoint supports MSI only.
• Endpoint does not support MSI-X.
• PCI_MSI_2 skips the device.
• Test may report no MSI vectors found / skip, even though an MSI-capable endpoint is present.
  Expected behavior:
• PCI_MSI_2 should accept endpoints that support either MSI or MSI-X.
• MSI-only endpoints should be included in the vector uniqueness validation.
• The test should skip only devices that support neither MSI nor MSI-X.
  Suggested fix:
• Update the capability check so the device is skipped only when both MSI-X and MSI capabilities are absent.
• Ensure the Linux PAL vector read path continues to handle both MSI-X table entries and MSI capability registers.
  Impact:
• MSI-only platforms cannot currently obtain automated ACS coverage for PCI_MSI_2.
• Such platforms may need manual validation until this is fixed.