usb: raw-gadget: fix null-ptr-deref when reenabling endpoints

Currently we preassign gadget endpoints to raw-gadget endpoints during
initialization. Fix resetting this assignment in raw_ioctl_ep_disable(),
otherwise we will get null-ptr-derefs when an endpoint is reenabled.

Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>