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: 8b645922 ("usb: gadget: Add support for USB MIDI 2.0 function driver")
Cc: stable <stable@kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20250904153932.13589-1-tiwai@suse.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>