Now this function uses frame data directly in the SPI transaction
instead making a copy of it.
Also fixing some length calculations and refactor/add tests to test this
function.
I like program with deny(clippy::pedantic) but it was set as allowed, so
I did get less linting errors/hints. Now it is corrected and also fix
the lint errors and hints.
Also fixes some comment and demagic some values.
Rename `FEC` to more appropriate name `FSC`.