Bluetooth: Fix possible deadlock in SCO code

sco_chan_del() only has conn != NULL when called from sco_conn_del() so
just move the code from it that deal with conn to sco_conn_del().

[  120.765529]
[  120.765529] ======================================================
[  120.766529] [ INFO: possible circular locking dependency detected ]
[  120.766529] 3.5.0-rc1-10292-g3701f94-dirty #70 Tainted: G        W
[  120.766529] -------------------------------------------------------
[  120.766529] kworker/u:3/1497 is trying to acquire lock:
[  120.766529]  (&(&conn->lock)->rlock#2){+.+...}, at:
[<ffffffffa00b7ecc>] sco_chan_del+0x4c/0x170 [bluetooth]
[  120.766529]
[  120.766529] but task is already holding lock:
[  120.766529]  (slock-AF_BLUETOOTH-BTPROTO_SCO){+.+...}, at:
[<ffffffffa00b8401>] sco_conn_del+0x61/0xe0 [bluetooth]
[  120.766529]
[  120.766529] which lock already depends on the new lock.
[  120.766529]
[  120.766529]
[  120.766529] the existing dependency chain (in reverse order) is:
[  120.766529]
[  120.766529] -> #1 (slock-AF_BLUETOOTH-BTPROTO_SCO){+.+...}:
[  120.766529]        [<ffffffff8107980e>] lock_acquire+0x8e/0xb0
[  120.766529]        [<ffffffff813c19e0>] _raw_spin_lock+0x40/0x80
[  120.766529]        [<ffffffffa00b85e9>] sco_connect_cfm+0x79/0x300
[bluetooth]
[  120.766529]        [<ffffffffa0094b13>]
hci_sync_conn_complete_evt.isra.90+0x343/0x400 [bluetooth]
[  120.766529]        [<ffffffffa009d447>] hci_event_packet+0x317/0xfb0
[bluetooth]
[  120.766529]        [<ffffffffa008aa68>] hci_rx_work+0x2c8/0x890
[bluetooth]
[  120.766529]        [<ffffffff81047db7>] process_one_work+0x197/0x460
[  120.766529]        [<ffffffff810489d6>] worker_thread+0x126/0x2d0
[  120.766529]        [<ffffffff8104ee4d>] kthread+0x9d/0xb0
[  120.766529]        [<ffffffff813c4294>] kernel_thread_helper+0x4/0x10
[  120.766529]
[  120.766529] -> #0 (&(&conn->lock)->rlock#2){+.+...}:
[  120.766529]        [<ffffffff81078a8a>] __lock_acquire+0x154a/0x1d30
[  120.766529]        [<ffffffff8107980e>] lock_acquire+0x8e/0xb0
[  120.766529]        [<ffffffff813c19e0>] _raw_spin_lock+0x40/0x80
[  120.766529]        [<ffffffffa00b7ecc>] sco_chan_del+0x4c/0x170
[bluetooth]
[  120.766529]        [<ffffffffa00b8414>] sco_conn_del+0x74/0xe0
[bluetooth]
[  120.766529]        [<ffffffffa00b88a2>] sco_disconn_cfm+0x32/0x60
[bluetooth]
[  120.766529]        [<ffffffffa0093a82>]
hci_disconn_complete_evt.isra.53+0x242/0x390 [bluetooth]
[  120.766529]        [<ffffffffa009d747>] hci_event_packet+0x617/0xfb0
[bluetooth]
[  120.766529]        [<ffffffffa008aa68>] hci_rx_work+0x2c8/0x890
[bluetooth]
[  120.766529]        [<ffffffff81047db7>] process_one_work+0x197/0x460
[  120.766529]        [<ffffffff810489d6>] worker_thread+0x126/0x2d0
[  120.766529]        [<ffffffff8104ee4d>] kthread+0x9d/0xb0
[  120.766529]        [<ffffffff813c4294>] kernel_thread_helper+0x4/0x10
[  120.766529]
[  120.766529] other info that might help us debug this:
[  120.766529]
[  120.766529]  Possible unsafe locking scenario:
[  120.766529]
[  120.766529]        CPU0                    CPU1
[  120.766529]        ----                    ----
[  120.766529]   lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
[  120.766529]
lock(&(&conn->lock)->rlock#2);
[  120.766529]
lock(slock-AF_BLUETOOTH-BTPROTO_SCO);
[  120.766529]   lock(&(&conn->lock)->rlock#2);
[  120.766529]
[  120.766529]  *** DEADLOCK ***

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>