Bluetooth: Fix LE connection timeout deadlock

The le_conn_timeout() may call hci_le_conn_failed() which in turn may
call hci_conn_del(). Trying to use the _sync variant for cancelling the
conn timeout from hci_conn_del() could therefore result in a deadlock.
This patch converts hci_conn_del() to use the non-sync variant so the
deadlock is not possible.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable@vger.kernel.org # 3.16.x