usb: gadget: midi2: Fix missing UMP group attributes initialization

The gadget card driver forgot to call snd_ump_update_group_attrs()
after adding FBs, and this leaves the UMP group attributes
uninitialized. As a result, -ENODEV error is returned at opening a
legacy rawmidi device as an inactive group.

This patch adds the missing call to address the behavior above.

Fixes: 8b645922b223 ("usb: gadget: Add support for USB MIDI 2.0 function driver")
Cc: stable <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/f_midi2.c

···
 			strscpy(fb->info.name, ump_fb_name(b),
 				sizeof(fb->info.name));
 		}
+		snd_ump_update_group_attrs(ump);
 	}
 
 	for (i = 0; i < midi2->num_eps; i++) {