staging: comedi: don't hijack hardware device private data

comedi_auto_config() associates a Comedi minor device number with an
auto-configured hardware device and comedi_auto_unconfig() disassociates
it.  Currently, these use the hardware device's private data pointer to
point to some allocated storage holding the minor device number.  This
is a bit of a waste of the hardware device's private data pointer,
preventing it from being used for something more useful by the low-level
comedi device drivers.  For example, it would make more sense if
comedi_usb_auto_config() was passed a pointer to the struct
usb_interface instead of the struct usb_device, but this cannot be done
currently because the low-level comedi drivers already use the private
data pointer in the struct usb_interface for something more useful.

This patch stops the comedi core hijacking the hardware device's private
data pointer.  Instead, comedi_auto_config() stores a pointer to the
hardware device's struct device in the struct comedi_device_file_info
associated with the minor device number, and comedi_auto_unconfig()
calls new function comedi_find_board_minor() to recover the minor device
number associated with the hardware device.

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