staging: comedi: reduce use of struct comedi_file_info

Since the parameters of `comedi_read_subdevice()` and
`comedi_write_subdevice()` have changed, there is no longer a need to
keep a local variable `struct comedi_file_info *info` hanging around in
the file operation functions and device attribute access functions.
Remove the variable from those function and replace the call sequence
`info = comedi_file_info_from_minor(minor)`,  `dev =
comedi_dev_from_file_info(info)` with the simpler `dev =
comedi_dev_from_minor(minor)`.

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>