staging: comedi: restrict comedi_set_hw_dev() usage

Don't allow comedi drivers to change `dev->hw_dev` using
`comedi_set_hw_dev()` if it's already been set.  Return `-EEXIST` in
that case.

`dev->hw_dev` needs to be set to NULL by the core during clean-up of the
comedi device, so add a local function `comedi_clear_hw_dev()` to do
that.

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