usb: gadget: udc: avoid dereference before NULL check in ep_queue

Coverity: CID 1260069

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>