Bluetooth: 6LoWPAN: add missing hci_dev_put() in get_l2cap_conn()

[ Upstream commit 747da130 ]

hci_get_route() takes reference, we should use hci_dev_put() to release
it when not need anymore.

Fixes: 6b8d4a6a ("Bluetooth: 6LoWPAN: Use connected oriented channel instead of fixed one")
Signed-off-by: Wang ShaoBo <bobo.shaobowang@huawei.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>