When the block number 0xff is given to Function Block Discovery
message, the device should return the information of all Function
Blocks, but currently the gadget driver treats it as an error.

Implement the proper behavior for the block 0xff instead.

Fixes: 8b645922 ("usb: gadget: Add support for USB MIDI 2.0 function driver")
Cc: stable@vger.kernel.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20240717095102.10493-1-tiwai@suse.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>