Channel distortion when converting S16_LE -> S32_LE ('plug' plugin)

[dh1dm]

When using a Terretec EWX2496 (ICE1712) a sample format conversion
is required in some cases as S32_LE is the only supported format.
This can be done by using the 'plug' plugin (e.g. 'plughw:0,0').
However, the left audio channel is distorted in a varying intensity,
it sounds like a resolution loss in the lower bits.
Applications with native S32_LE support (e.g. Audacity) do not show this
effect.
I'd suppose that the responsible code is in 'alsa-lib/src/pcm/pcm_plug.c',
but up to now I could not track down this issue.

[dh1dm]

As the PCI bus is very rare nowadays, the remaining PCIe <-> PCI bridge based designs
are somewhat a HW niche. It is up to now not clear if the distortion is definitely generated
by the ALSA lib or if it is a bridge resource allocation or ASPM problem.
I've set CONFIG_PCIEASPM_PERFORMANCE=y and CONFIG_PCI_REALLOC_ENABLE_AUTO=y
and in a short test everything seems ok. However, it is not clear if this is a final solution.