cxgbe(4): Changes to ULD list management.

[np]

Summary

• Convert t4_uld_list to an array. There will be at most 3 items in the list and it's simpler to track them in an array with a fixed slot for each ULD.
• There is no need to refcount ULDs so stop doing that.
• Add uld_ prefix to all members of uld_info.
• Rename async_event to uld_stop to match its actual purpose. Call it for all ULDs and not just ULD_IWARP.