usb: gadget: u_audio: Clear uac pointer when freed.

This prevents use of a stale pointer if functions are called after
g_cleanup that shouldn't be. This doesn't fix any races, but converts
a possibly silent kernel memory corruption into an obvious NULL pointer
dereference report.

Fixes: eb9fecb9

 ("usb: gadget: f_uac2: split out audio core")
Signed-off-by: Chris Wulff <chris.wulff@biamp.com>
Link: https://lore.kernel.org/stable/CO1PR17MB54194226DA08BFC9EBD8C163E1172%40CO1PR17MB5419.namprd17.prod.outlook.com
Link: https://lore.kernel.org/r/CO1PR17MB54194226DA08BFC9EBD8C163E1172@CO1PR17MB5419.namprd17.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>