staging: comedi: ni_tiocmd: pass subdevice to command setup functions

The `ni_tio_input_cmd()`, `ni_tio_output_cmd()` and `ni_tio_cmd_setup()`
functions are called from `ni_tio_cmd()` to set up a comedi command.
They currently get passed two parameters, a pointer to a `struct
ni_gpct` and a pointer to a `struct comedi_async`, but both of those
come from members of a `struct comedi_subdevice` (the former from the
`private` member).  Replace the parameters with a pointer to the `struct
comedi_subdevice`.

The main motive is to make the comedi subdevice more easily available to
the functions for the use of subsequent patches to remove the `struct
comedi_async *` parameters from the comedi buffer handling functions.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>