n_tty: make many tty parameters const

In many n_tty functions, the 'tty' parameter is used to either obtain
'ldata', or test the tty flags. So mark 'tty' in them const to make
obvious that it is only read.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230712064216.12150-5-jirislaby@kernel.org


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