CFG_ERR, DATA_UNDERRUN and DELIM_UNDERRUN are all flags, rather than part of ts_status. Thus:
CFG_ERR, DATA_UNDERRUN and DELIM_UNDERRUN are all flags, rather than
part of ts_status. Thus:

* make sure we decode them from ts_flags, rather than ts_status;
* make sure we decode them regardless of whether there's an error or not.

This correctly exposes descriptor configuration errors, TX delimiter
underruns and TX data underruns.

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