oxfw: Add support for capture/playback MIDI messages

I assume that 'if the device has MIDI comformant data channels in its
stream formation, the device has one MIDI port'. With this assumption,
this driver create MIDI devices.

When no streams have already started, MIDI functionality starts stream
with current sampling rate.

When MIDI functionality has already starts some streams and PCM
functionality is going to start streams at different sampling rate,
this driver stops streams once and changes sampling rate, then restarts
streams.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
oxfw: Add hwdep interface

This interface is designed for mixer/control application. To use hwdep
interface, the application can get information about firewire node, can
lock/unlock kernel streaming and can get notification at starting/stopping
kernel streaming.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>