staging: comedi: change comedi_alloc_subdevice_minor parameters

`comedi_alloc_subdevice_minor()` doesn't really need a parameter
pointing to a `struct comedi_device` as it can get this information from
the parameter pointing to a `struct comedi_subdevice`.  Just pass the
subdevice parameter.

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