sound fixes for 4.16-rc7

Things look calming down, but people were still busy to plaster over
small holes:

- Two fixes to harden against races in aloop driver
- A correction of a long-standing bug in USB-audio UAC2 processing
  unit parser
- As usual suspects, HD-audio: a workaround for Coffee Lake
  controller and a few other device-specific fixes

All small and for stable.
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