usb: gadget: u_audio: Fix race condition use of controls after free during gadget unbind.

Hang on to the control IDs instead of pointers since those are correctly
handled with locks.

Fixes: 8fe9a03f ("usb: gadget: u_audio: Rate ctl notifies about current srate (0=stopped)")
Fixes: c565ad07 ("usb: gadget: u_audio: Support multiple sampling rates")
Fixes: 02de698c

 ("usb: gadget: u_audio: add bi-directional volume and mute support")
Signed-off-by: Chris Wulff <chris.wulff@biamp.com>
Link: https://lore.kernel.org/stable/CO1PR17MB5419C2BF44D400E4E620C1ADE1172%40CO1PR17MB5419.namprd17.prod.outlook.com
Link: https://lore.kernel.org/r/CO1PR17MB5419C2BF44D400E4E620C1ADE1172@CO1PR17MB5419.namprd17.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>