usb: wusbcore: avoid stack overflow in URB enqueue error path

This patch modifies wa_urb_enqueue to return an error and not call the
urb completion routine if it failed to enqueue the urb because the HWA
device is gone.  This prevents a stack overflow due to infinite
submit/complete recursion when unplugging the HWA while connected to a
HID device.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>