staging: comedi: remove hardware_device from struct comedi_device_info

The `hardware_device` member of `struct comedi_device_info` is only set
to point to a hardware device by the auto-configuration code (specifically
`comedi_alloc_board_minor() with a non-NULL `hardware_device` parameter)
so that it can be found again by the auto-unconfiguration code
(specifically `comedi_release_hardware_device()`).  However, as
`info->device->hw_dev` (where `info` is a pointer to the `struct
comedi_device_info`) is set to the same value as `info->hardware_device`
(by calling `comedi_set_hw_dev()` with the same hardware device pointer)
we can look for that instead, so there is no need for the
`hardware_device` member any more.  Get rid of it.

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>