asix-sigma: re-enable software check for exact trigger position

asix-sigma: re-enable software check for exact trigger position

The hardware provided trigger match location is inaccurate. Do check
sample values against the initial trigger condition spec for a short
range of the retrieved sample data, to refine the trigger marker's
position which is sent to the session feed.

Temporarily ignore the optional sample count limit for trigger-using
acquisitions, to reduce the diff size and simplify review. Since the
hardware transparently compresses sample data, we cannot reliably
determine where to start the download and interpretation of sample data,
and the submission to the session feed. Starting early in the sample
memory content, and sticking with the strict sample count limit, could
clip submission before the actual trigger position.

This implementation provides _at least_ the requested amount of data,
and does cover the spot of interest (the trigger position). This, and
the trigger support's having become operational again, is considered an
important improvement. The inaccuracy is considered acceptable for now.
Trigger-less acquisition does enforce the exact sample count limit.