asix-sigma: rework outer sample download loop (DRAM lines iteration)

Factor USB data transfer out of the code path which interprets sample
memory content. Keep internal state of sample memory download in the
device context. This eliminates local variables, and ideally allows a
future implementation to spread chunked downloads across several read
callbacks, which would improve UI responsiveness.

Update comments while we are here. Address minor portability nits (ull
suffix vs UINT64_C macro). The inner loops (iterating clusters and their
events which contain individual samples) are not affected by this commit.