dgnc: remove old 2.4-2.6 compat kernel defines

dgnc_kcompat.h contains some old legacy defines in case the kernel
doesn't have __user defined but for current kernel versions these
defines don't make sense and are useless so remove them. Move the
TTY_FLIPBUF_SIZE define to digi.h because it's used in the code.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>