serial_io.py: simplify cmd() method

According to the research in Issue #60 (https://github.com/DFE/MONK/issues/60),
the simplest implementation for best case scenarios is only 2 lines. This
commit implements those 2 lines instead of the complex behaviour before.

Issue #69 (https://github.com/DFE/MONK/issues/69) shall improve or replace that
method with a more practical implementation, if that is not already finished
with Issue #61 (https://github.com/DFE/MONK/issues/61).

See Github Issue #60 for details.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Acked-by: Steffen Sledz <sledz@dresearch-fe.de>
Acked-by: Eik Binschek <binschek@dresearch-fe.de>