Bluetooth: Fix possible use after free in delete path

We need to use the _sync() version for cancelling the info and security
timer in the L2CAP connection delete path. Otherwise the delayed work
handler might run after the connection object is freed.

Signed-off-by: Ulisses Furquim <ulisses@profusion.mobi>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>