kingst-la2016: experiment with USB bulk size and pool count

kingst-la2016: experiment with USB bulk size and pool count

The previous implementation used a pool of 32 USB bulk transfers of up
to 256KiB size each. Using a pool did speedup capture data downloads,
using more transfers or larger buffers would not make a difference.
Slightly reduce the total memory consumption again, change the setup to
8 transfers of up to 512KiB each, which results in a total of 4MiB for
pending USB communication. These numbers are as arbitrary as any other.
No attempt was made to automatically derive the values from acquisition
parameters like samplerate and samples count.