asix-sigma: Acquisition stop, symbolic identifiers for mode register fields

asix-sigma: Acquisition stop, symbolic identifiers for mode register fields

Enhance how the data acquisition is stopped. Wait for the hardware to
flag the successful completion of data retrieval as well as flushing
through hardware pipelines.

Use symbolic identifiers for the mode register's fields (for read as
well as write access).

This commit uses part of a code update to better match the documentation
done by jry@, but not all of it to reduce the size of the commit.