staging: comedi: export comedi_set_hw_dev()

Chnage the inline `comedi_set_hw_dev()` to an exported function and
change it's return type from `void` to `int` so we can impose some
restrictions (in a later patch) and return an error if necessary.

Only a few comedi drivers call this, although they don't need to if the
hardware device has been attached automatically via
`comedi_auto_config()` and the comedi driver's `auto_attach()` method.

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