ALSA: oxfw: support more MIDI ports

In IEC 61883-6, sequence multiplexing is applied to MIDI conformant data
channel. As a result, eight MIDI data streams are included in the channel.
Although ALSA AM824 data block processing layer implements this
multiplexing, current OXFW driver doesn't utilize it due to wrong
calculation of MIDI ports.

This commit fixes this bug to add proper calculation. Although this commit
allows to use 8 MIDI data streams, the number of available MIDI ports is
limited by the number of ALSA MIDI ports added by the driver.

Fixes: df075fee('ALSA: firewire-lib: complete AM824 data block processing layer')
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>