asix-sigma: Factor out access to sample data and session data

asix-sigma: Factor out access to sample data and session data

Introduce helper routines to access the sample data that is provided by
the ASIX hardware, as well as the buffer which accumulates logic data
before it gets sent to the session's datafeed.

This hides endianess issues from call sites, and prepares access to
memory layout which varies with sample frequencies.

This commit is based on work done by jry@.