usb: gadget: avoid exposing kernel stack

Function in_rq_cur copies random bytes from the stack.
Zero the memory instead.

Fixes: 132fcb46 ("usb: gadget: Add Audio Class 2.0 Driver")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>