dgnc: clean up digi.h

Remove a lot of unused structs and defines from digi.h. We still have to
be careful with TIOCM_LE and TIOCMSET/TIOCMBIC because termios.h and
ioctls.h respectfully redefine them.
Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>