asix-sigma: Fixup the download of the last data acquisition chunk

Correctly determine the size of a download chunk for the last DRAM row
that's involved in the recent data acquisition.

This commit is based on work done by jry@.

This addresses bug #838 (trailing garbage).

It's assumed that the previously downloaded excess data was "swallowed"
by the sample count enforcement logic that was applied earlier, so the
(remainder of the) issue could have gone unnoticed, unless some other
termination condition than sample count was used.