n_tty: make n_tty_receive_char_special return void

After the previous patch, noone cares about the return value of
n_tty_receive_char_special. ldata->lnext is checked instead.

So switch return type of n_tty_receive_char_special to void.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-8-jslaby@suse.cz


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