usb: gadget: at91udc: Don't check for ep->ep.desc

Earlier we used to check for ep->ep.desc to figure out if this ep has
already been enabled and if so, abort.
Ido Shayevitz removed the usb_endpoint_descriptor from private udc
structure 5a6506f0 ("usb: gadget: Update at91_udc to use
usb_endpoint_descriptor	inside the struct usb_ep") but did not fix up
the ep_enable condition because _now_ the member is always true and we
can't check if this ep is enabled twice.

Cc: Ido Shayevitz <idos@codeaurora.org>
Tested-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Tested-by: Mario Isidoro <Mario.Isidoro@tecmic.pt>
Cc: <stable@kernel.org> # v3.5
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>