GCS_MAVLink: correct resetting of parity after passthhru is done

[peterbarker]

this might have worked if parity1 and parity2 were static values - but theyr're not. I'm guessing the code evolved

../../libraries/GCS_MAVLink/GCS_Common.cpp:6847:29: warning: Although the value stored to 'parity1' is used in the enclosing expression, the value is never actually read from 'parity1' [deadcode.DeadStores]
        _passthru.parity1 = parity1 = _passthru.port1->get_parity();
                            ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../libraries/GCS_MAVLink/GCS_Common.cpp:6848:29: warning: Although the value stored to 'parity2' is used in the enclosing expression, the value is never actually read from 'parity2' [deadcode.DeadStores]
        _passthru.parity2 = parity2 = _passthru.port2->get_parity();
                            ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~