usb: gadget: Clear usb_endpoint_descriptor inside the struct usb_ep on disable

This fix a bug in f_serial, which expect the ep->desc to be NULL after
disabling an endpoint.

Cc: stable@vger.kernel.org
Signed-off-by: Ido Shayevitz <idos@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>