USB: symbolserial: Use usb_get_serial_port_data

USB: symbolserial: Use usb_get_serial_port_data

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: a85796ee5149 ("USB: symbolserial: move private-data allocation to
port_probe")
Cc: stable <stable@vger.kernel.org>     # v3.10
Acked-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>