Bluetooth: Fix for ACL disconnect when pairing fails

When pairing fails hci_conn refcnt drops below zero. This cause that
ACL link is not disconnected when disconnect timeout fires.

Probably this is because l2cap_conn_del calls l2cap_chan_del for each
channel, and inside l2cap_chan_del conn is dropped. After that loop
hci_chan_del is called which also drops conn.

Anyway, as it is desrcibed in hci_core.h, it is known that refcnt
drops below 0 sometimes and it should be fine. If so, let disconnect
link when hci_conn_timeout fires and refcnt is 0 or below. This patch
does it.

This affects PTS test SM_TC_JW_BV_05_C

Logs from scenario:

[69713.706227] [6515] pair_device:
[69713.706230] [6515] hci_conn_add: hci0 dst 00:1b:dc:06:06:22
[69713.706233] [6515] hci_dev_hold: hci0 orig refcnt 8
[69713.706235] [6515] hci_conn_init_sysfs: conn ffff88021f65a000
[69713.706239] [6515] hci_req_add_ev: hci0 opcode 0x200d plen 25
[69713.706242] [6515] hci_prepare_cmd: skb len 28
[69713.706243] [6515] hci_req_run: length 1...