Bluetooth: 6lowpan: remove unnecessary codes in give_skb_to_upper

netif_rx() only returns NET_RX_DROP and NET_RX_SUCCESS, not returns
negative value

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>