tty: refactor TIOCSSERIAL handling

Factor out the deprecated serial flags handling and tty-operation check
shared with the compat TIOCSSERIAL handler.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210407095208.31838-6-johan@kernel.org


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