staging/comedi: keep reference to class device after destroyed

staging/comedi: keep reference to class device after destroyed

When a dynamically allocated `struct comedi_device` gets automatically
unconfigured by a call to `comedi_auto_unconfig()` from a lower-level
driver's bus removal function (e.g. when a USB device is disconnected),
the class device in `dev->class_dev` (where `dev` points to the `struct
comedi_device`) is destroyed by a call to `device_destroy()` that
matches a previous call to `device_create()`.

However, if the `struct comedi_device` is still associated with an open
file object, the now invalid `dev->class_dev` pointer may still be
passed to `dev_printk()` (via `dev_dbg()` etc.), producing bogus output
or worse.

To fix this, call `get_device()` on the class device if
`device_create()` was successful.  Add a matching call to `put_device()`
in `comedi_dev_kref_release()` when the `struct comedi_device` is freed.
The calls to `dev_dbg()` etc. after the call to `device_destroy()` will
still produce valid output, although the device will have been
unregistered in sysfs.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>