Bluetooth: Add support for tracking transmitted packets

Protocols/profiles may want to know when exactly their PDU has been
transmitted over the air. To make this possible, introduce support for
a callback that will get called when the controller reports that a
packet has been transmitted (through the Number of Completed Packets
HCI event).

Change-Id: Ia3a19b93c5b2111f144bfabe5861187c41525f30
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>