Bluetooth: Make use of skb_pull to parse L2CAP signaling PDUs

This uses skb_pull when parsing signalling PDUs so skb->data for
pointing to the current PDU and skb->len as the remaining bytes to be
processed.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>