netdev_driver_t::recv: Fixed preconditions in doc

Removed the invalid precondition in netdev_driver_t::recv that the buffer never
NULL and the length positive. (In the "get message size" case both is wrong, in
the drop message case the first part is wrong.)

drivers/include/net/netdev.h

@@ -313,7 +313,6 @@ typedef struct netdev_driver {
      * @brief Get a received frame
      *
      * @pre `(dev != NULL)`
-     * @pre `(buf != NULL) && (len > 0)`
      *
      * Supposed to be called from
      * @ref netdev_t::event_callback "netdev->event_callback()"