usb: musb: gadget: fix list_head usage

commit ad1adb89


(usb: musb: gadget: do not poke with gadget's list_head)
fixed a bug in musb where it was corrupting the list_head
which is supposed to be used by gadget drivers. While
doing that, I forgot to fix the usage in musb_gadget_dequeue()
method. Fix that.

Reported-by: Pavol Kurina <pavol.kurina@emsys.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>