Bluetooth: ATT: Fix not returning error

bt_l2cap_send_cb may fail if there are no context available which means
that the request would not be sent, also due to the use of custom
callback it cannot be queued either so the only option is to return the
error and let the application handle it.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>