plugins/dmxusb: fixed QtSerialPort usage
plugins/dmxusb: fixed QtSerialPort usage

There's still some object created in the wrong thread, but apparently it works

kripton/qlcplus,sbenejam/qlcplus,sbenejam/qlcplus,sbenejam/qlcplus,joepadmiraal/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,plugz/qlcplus,sbenejam/qlcplus,joepadmiraal/qlcplus,joepadmiraal/qlcplus,sbenejam/qlcplus,plugz/qlcplus,kripton/qlcplus,plugz/qlcplus,sbenejam/qlcplus,nedmech/qlcplus,mcallegari/qlcplus,nedmech/qlcplus,mcallegari/qlcplus,kripton/qlcplus,plugz/qlcplus,plugz/qlcplus,kripton/qlcplus,joepadmiraal/qlcplus,joepadmiraal/qlcplus,plugz/qlcplus,plugz/qlcplus,nedmech/qlcplus,kripton/qlcplus,sbenejam/qlcplus,nedmech/qlcplus,kripton/qlcplus,mcallegari/qlcplus,joepadmiraal/qlcplus,nedmech/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,nedmech/qlcplus,kripton/qlcplus