Bluetooth: Fix wrong module refcount when connection setup fails

The module refcount is increased by hci_dev_hold() call in hci_conn_add()
and decreased by hci_dev_put() call in del_conn(). In case the connection
setup fails, hci_dev_put() is never called.

Procedure to reproduce the issue:

  # hciconfig hci0 up
  # lsmod | grep btusb                   -> "used by" refcount = 1

  # hcitool cc <non-exisiting bdaddr>    -> will get timeout

  # lsmod | grep btusb                   -> "used by" refcount = 2
  # hciconfig hci0 down
  # lsmod | grep btusb                   -> "used by" refcount = 1
  # rmmod btusb                          -> ERROR: Module btusb is in use

The hci_dev_put() call got moved into del_conn() with the 2.6.25 kernel
to fix an issue with hci_dev going away before hci_conn. However that
change was wrong and introduced this problem.

When calling hci_conn_del() it has to call hci_dev_put() after freeing
the connection details. This handling should be fully symmetric. The
execution of del_conn() is done in a work queue and needs it own calls
to hci_dev_hold() and hci_dev_put() to ensure that the hci_dev stays
until the connection cleanup has been finished.

Based on a report by Bing Zhao <bzhao@marvell.com>

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Tested-by: Bing Zhao <bzhao@marvell.com>