asix-sigma: Enforce optionally specified sample count

asix-sigma: Enforce optionally specified sample count

The Asix Sigma hardware does not support a sample count limit. Instead
this optional input parameter gets mapped to a sample time, and some
slack for hardware pipelines and compression gets added. When data
acquisition completes and sample data gets downloaded, chances are that
there is more data than requested by the user.

Do enforce the optional sample count limit. Stop sending data to the
sigrok session when the configured number of samples was sent.

This commit is based on work done by jry@.

This fixes bug #838.