USB: symbolserial: Use usb_get_serial_port_data

commit 951d3793bbfc0a441d791d820183aa3085c83ea9 upstream.

The driver used usb_get_serial_data(port->serial) which compiled but resulted
in a NULL pointer being returned (and subsequently used). I did not go deeper
into this but I guess this is a regression.

Signed-off-by: Philipp Hachtmann <hachti@hachti.de>
Fixes: a85796ee ("USB: symbolserial: move private-data allocation to
port_probe")
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>