kingst-la2016: Eliminate non-portable packed structs for USB transfer layout

Eliminate packed structs in the construction and in the reception of
USB transfers, since these are rather non-portable. Use common support
for endianess aware access to byte streams instead. This unbreaks the
driver for MinGW builds, and increases portability and maintainability
on all other sigrok supported platforms.

[ gsi: adjust more locations, improve robustness, fixup style, reword message ]