cxgbe(4): Changes to ULD list management.

• 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.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D46029