To make it clearer which legacy substream corresponds to which UMP
group, fill the subname field of each substream object with the group
number and the endpoint name, e.g. "Group 1 (My Device)".

Ideally speaking, we should have some better link information to the
derived UMP, but it's another feature extension.

Fixes: 0b5288f5 ("ALSA: ump: Add legacy raw MIDI support")
Link: https://lore.kernel.org/r/20230824075108.29958-3-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>