ALSA: oxfw: calculating MIDI ports in stream discover

Current OXFW driver calculates the number of MIDI ports just before adding
ALSA MIDI ports. It's convenient for some devices with quirks to move
these codes before handling quirks.

This commit implements this idea.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>