tty: n_tty: move canon handling to a separate function

n_tty_receive_char_special() is already complicated enough. Split the
canon handling to a separate function: n_tty_receive_char_canon().

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230827074147.2287-8-jirislaby@kernel.org


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