usb: gadget: network: fix bind() error path

I think this is wrong since 72c973dd ("usb: gadget: add
usb_endpoint_descriptor to struct usb_ep"). If we fail to allocate an ep
or bail out early we shouldn't check for the descriptor which is
assigned at ep_enable() time.

Cc: Tatyana Brokhman <tlinder@codeaurora.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>