Bluetooth: Fix error code in chan_alloc_skb_cb()

[ Upstream commit 72d061ee630d0dbb45c2920d8d19b3861c413e54 ]

The chan_alloc_skb_cb() function is supposed to return error pointers on
error.  Returning NULL will lead to a NULL dereference.

Fixes: 6b8d4a6a ("Bluetooth: 6LoWPAN: Use connected oriented channel instead of fixed one")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>