n_tty: drop parmrk_dbl from n_tty_receive_char

After the previous cleanup patches, parmrk_dbl parameter is always true
-- I_PARMRK is checked only in n_tty_receive_char now. So remove
parmrk_dbl completely.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-6-jslaby@suse.cz


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