Bluetooth: Fix freeing up conn->tx_notify upon disconnect

It's possible that tx_notify still contains items when a Disconnect
Complete happens. Since the normal path for processing tx_notify is
not taken when the connection is not in CONNECTED state, we must make
sure to process the list latest in conn_cleanup() that's called as one
last thing before the connection object is freed up.

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