n_hdlc: remove unneeded ifdef

TTY_NO_WRITE_SPLIT is (always) defined in linux/tty.h, so no need to
check for it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-18-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>