Fix indentation

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Introduce private header and reorganize the code

* make a p->device just a pointer not a buffer
* move everything private to the private header
* move set_device() function to common source file rs232.c
* move platform dependent defines to the corresponding files as they're
  private anyway

Signed-off-by: Petr Štetiar <ynezz@true.cz>