ALSA: ump: Fill group names for legacy rawmidi substreams

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