ALSA: usb-audio: us144mkii: Add MIDI support and mixer controls

This patch adds the remaining features to the driver: MIDI I/O and
ALSA mixer controls.

A raw MIDI device is created, supporting one input and one output port.
The implementation handles the device-specific 9-byte packet format for
both sending and receiving MIDI data, using bulk URBs and workqueues for
processing.

ALSA kcontrols are added to the mixer interface, allowing user-space
control over the audio routing matrix (e.g., selecting analog/digital
sources for capture channels, and routing playback streams to different
outputs). A read-only control to display the current sample rate is
also included.

Finally, the stub routing functions are replaced with the full logic
to apply the user-selected mixer settings to the audio streams.

With this commit, the driver is feature-complete.

Signed-off-by: Šerif Rami <ramiserifpersia@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://patch.msgid.link/20250814172222.9448-6-ramiserifpersia@gmail.com