staging: comedi: use file->private_data in file operations

Since the `struct comedi_device` should now be protected from being
freed while an open file object is using it, use the `private_data`
member of the `struct file` to point to it.  Set it in `comedi_open()`
and use it in the other file operation handlers instead of calling
`comedi_dev_from_minor()` and checking the result.

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