asix-sigma: store "limit samples" value, re-determine "limit msecs" period

The driver internally implements the "limit samples" feature by means of
the "limit sample period" approach. Determination of the corresponding
period of time for captures depends on the sample rate as well as the
maximum sample count, and thus needs to be re-done when either setting
changes.

Introduce a "limit_samples" variable so that the value is available when
needed later. As a byproduct the parameter can be retrieved now (get).

Add comments to the sigma_set_samplerate() routine's sections, since
quite a bit is happening there, and interacts with other locations.

Signed-off-by: Gerhard Sittig <redacted>