USB: serial: mos7840: drop port open flag

Drop the redundant port open flag and corresponding checks. USB serial
core will not call any of these driver callbacks for a closed port, and the
write URBs are stopped at close().

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>