ALSA: usb-audio: Attach legacy rawmidi after probing all UMP EPs

The legacy rawmidi devices are the shadows of the main UMP devices,
hence it's better to initialize them after all UMP Endpoints are
parsed.  Then, at the moment the legacy rawmidi is created, we already
know the static flag or the proper EP name string, and we can fill
those information at UMP core side instead of fiddling the attributes
at a later point.

Fixes: ec362b63 ("ALSA: usb-audio: Enable the legacy raw MIDI support")
Link: https://lore.kernel.org/r/20230824075108.29958-2-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>