usb: gadget: mv_u3d_core: fix violation of locking discipline in mv_u3d_ep_disable()

mv_u3d_nuke() expects to be calles with ep->u3d->lock held,
because mv_u3d_done() does. But mv_u3d_ep_disable() calls it
without lock that can lead to unpleasant consequences.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Felipe Balbi <balbi@ti.com>