read_fifo() used part of the frame buffer to readout non-frame data.
This results in incorrect readout of the fifo buffer but also the full
MTU could not be used.
Also added some more tests to check this and that the readout is a
multipule of 4 bytes.
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`.
