Bluetooth: ATT: Handle Confirmations

This adds the callback necessary to parse Confirm PDU:

< ACL Data TX: Handle 3585 flags 0x00 dlen 8
      ATT: Handle Value Indication (0x1d) len 3
        Handle: 0x001d
          Data: 00
> ACL Data RX: Handle 3585 flags 0x02 dlen 5
      ATT: Handle Value Confirmation (0x1e) len 0

Change-Id: I870058150f27718541eae959dcd5d9d7c6414ad3
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>