usb: gadget: clean the ep in autoconf before returning it.

Since commit 72c973dd aka ("usb: gadget: add usb_endpoint_descriptor to
struct usb_ep) the descriptor is part of the ep. Most gadgets like
g_zero or masstorage call config_ep_by_speed() to grab an available
endpoint which may be used for FS/HS/SS bulk/iso/intr and in a second
they assign the proper descriptor by calling config_ep_by_speed(). This
is good so far. A few of them like ncm call config_ep_by_speed() only if
ep->desc not assigned earlier. That means ep->desc is never assigned if
the endpoint was used by another gadget before it was removed.

Some of those gadgets also assign ep->driver_data to NULL on reset or
ep_disable part _but_ keep a reference to this endpoint. At ep_enable
time they assign driver_data to their private data. This probably needs
a clean up of its own.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>