ALSA: bebob: detect the number of available MIDI ports

Current implementation counts the number of input/output plugs for MIDI
type and uses the count as the number of physical MIDI ports. However,
the number of channels of the port represents the count.

This commit fixes the bug by additional vendor-specific AVC command
extension.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210321032831.340278-3-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>