Bluetooth: SMP: Track when last key distribution PDU has been sent

According to the SMP specification the pairing is only to be
considered complete once the last SMP PDU has been transmitted over
the air. Take advantage of the new TX callback to notify completion
only once the packet has really been transmitted.

Change-Id: Ic87e598cd0e040d99f38344b98e476f67e4d9762
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>