staging: comedi: return void from comedi_driver_unregister()

'Unregister' functions generally return `void`.
`comedi_driver_unregister()` currently returns an `int` errno value.
Nothing looks at the return value.  Change the return type to `void`.

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