Fix a "loop variable creates a copy" warning

kmix/backends/mixer_alsa9.cpp: In member function
    'virtual int Mixer_ALSA::readVolumeFromHW(const QString&, std::shared_ptr<MixDevice>)':
kmix/backends/mixer_alsa9.cpp:846: warning: loop variable 'md' creates
     a copy from type 'const std::shared_ptr<MixDevice>'
kmix/backends/mixer_alsa9.cpp:846: note: use reference type to prevent copying