Bluetooth: Fix race between hci_register_dev() and hci_dev_open()

Bluetooth: Fix race between hci_register_dev() and hci_dev_open()

If hci_dev_open() is called after hci_register_dev() added the device to
the hci_dev_list but before the workqueue are created we could run into a
NULL pointer dereference (see below).

This bug is very unlikely to happen, systems using bluetoothd to
manage their bluetooth devices will never see this happen.

BUG: unable to handle kernel NULL pointer dereference
0100
IP: [<ffffffff81077502>] __queue_work+0x32/0x3d0
(...)
Call Trace:
 [<ffffffff81077be5>] queue_work_on+0x45/0x50
 [<ffffffffa016e8ff>] hci_req_run+0xbf/0xf0 [bluetooth]
 [<ffffffffa01709b0>] ? hci_init2_req+0x720/0x720 [bluetooth]
 [<ffffffffa016ea06>] __hci_req_sync+0xd6/0x1c0 [bluetooth]
 [<ffffffff8108ee10>] ? try_to_wake_up+0x2b0/0x2b0
 [<ffffffff8150e3f0>] ? usb_autopm_put_interface+0x30/0x40
 [<ffffffffa016fad5>] hci_dev_open+0x275/0x2e0 [bluetooth]
 [<ffffffffa0182752>] hci_sock_ioctl+0x1f2/0x3f0 [bluetooth]
 [<ffffffff815c6050>] sock_do_ioctl+0x30/0x70
 [<ffffffff815c75f9>] sock_ioctl+0x79/0x2f0
 [<ffffffff811a8046>] do_vfs_ioctl+0x96/0x560
 [<ffffffff811a85a1>] SyS_ioctl+0x91/0xb0
 [<ffffffff816d989d>] system_call_fastpath+0x1a/0x1f

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>