tty: use assign_bit() in port-flag accessors

Use the new assign_bit() wrapper in the port-flag accessors instead of
open coding.

Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20201202113942.27024-2-johan@kernel.org


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