staging: comedi: remove comedi_file_info_from_board_minor()

Merge the code from `comedi_file_info_from_board_minor()` into
`comedi_dev_from_board_minor()` and remove it since nothing else calls
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>