tty: use const parameters in port-flag accessors

Declare the port parameter to the flag-test accessors as const.

This is currently mostly cosmetic as the accessors are already inlined.

Suggested-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20201202113942.27024-3-johan@kernel.org


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