ALSA: usb-audio: Enable the legacy raw MIDI support

Attach the legacy rawmidi devices when enabled in Kconfig accordingly.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-16-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>