staging: comedi: use refcount in comedi_driver_unregister()

Change `comedi_driver_unregister()` to call
`comedi_dev_get_from_minor()` instead of `comedi_dev_from_minor()` when
finding devices using the driver.  This increments the reference count
to prevent the device being removed while it is being checked to see if
it is attached to the driver.  Call `comedi_dev_put()` to decrement the
reference afterwards.

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