soi: Don't call DMA sync API when not needed

Merge series from Andy Shevchenko <andriy.shevchenko@linux.intel.com>:

A couple of fixes to avoid calling DMA sync API when it's not needed.
This doesn't stop from discussing if IOMMU code is doing the right thing,
i.e. dereferences SG list when orig_nents == 0, but this is a separate
story.