staging: comedi: remove comedi_dev_from_minor()

The `comedi_dev_from_minor()` function is no longer used, so remove it.
Calls to it have either been replaced by calls to
`comedi_dev_get_from_minor()` or by using the `private_data` member of
the open file object.

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