USB: pl2303: disable break on shutdown

Currently an enabled break state is not disabled on final close nor on
re-open and has to be disabled manually.

Fix this by disabling break on port shutdown.

Reported-by: Jari Ruusu <jariruusu@users.sourceforge.net>
Tested-by: Jari Ruusu <jariruusu@users.sourceforge.net>
Signed-off-by: Johan Hovold <johan@kernel.org>