asix-sigma: Handle sample memory wrap around (circular buffer)

asix-sigma: Handle sample memory wrap around (circular buffer)

Handle the case when the sample data memory was filled and has wrapped
around during acquisition. Download the respective part of the data
which is reliably available, only skipping a single 1KB row which might
contain either old or new data while it's not certain which it would be.

This will be essential when triggers later become available. Right now
it copes with user requests for sample counts that exceed the total DRAM
capacity. Instead the maximum available amount of data is provided.

Of course acquisition no longer gets stopped when the end of DRAM is
reached.