sound fixes for 4.15-rc1

All commits found here are small fixes for regression or stable:
- PCM timestamp behavior fix that could be seen as a regression
- Remove spurious WARN_ON() from ALSA timer 32bit compat ioctl
- HD-audio HDMI/DP channel mapping fix for 32bit archs
- Fix the previous fix for HD-audio initialization code
- More hardening USB-audio against malicious USB descriptors
- HD-audio quirks/fixes (Realtek codec, AMD controller)
- Missing help text for the recent Intel SST kconfig change
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