ALSA: usb-audio: Create UMP blocks from USB MIDI GTBs

USB MIDI spec defines the Group Terminal Blocks (GTB) that associate
multiple UMP Groups.  Those correspond to snd_ump_block entities in
ALSA UMP abstraction, and now we create those UMP Block objects for
each UMP Endpoint from the parsed GTB information.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-13-tiwai@suse.de


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