usb: gadget: fix wrong endpoint desc

Gadget driver should always use config_ep_by_speed() to initialize
usb_ep struct according to usb device's operating speed. Otherwise,
usb_ep struct may be wrong if usb devcie's operating speed is changed.

The key point in this patch is that we want to make sure the desc pointer
in usb_ep struct will be set to NULL when gadget is disconnected.
This will force it to call config_ep_by_speed() to correctly initialize
usb_ep struct based on the new operating speed when gadget is
re-connected later.

Reviewed-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: EJ Hsu <ejh@nvidia.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>