ALSA: bebob: enable to deliver MIDI messages for multiple ports

Current implementation of bebob driver doesn't correctly handle the case
that the device has multiple MIDI ports. The cause is the number of MIDI
conformant data channels is passed to AM824 data block processing layer.

This commit fixes the bug.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210321032831.340278-4-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>