Bluetooth: Fix error code in chan_alloc_skb_cb()

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>