staging: comedi: refactor comedi_dev_from_minor()

Refactor `comedi_dev_from_minor()` to call one of two new functions
`comedi_dev_from_board_minor()` (for minor device numbers less than
`COMEDI_NUM_BOARD_MINORS`) or `comedi_dev_from_subdevice_minor()` (for
minor device numbers greater than or equal to `COMEDI_NUM_BOARD_MINORS`,
which are subdevice minor device numbers).

Remove `comedi_file_info_from_minor()` as it is no longer used.

This is a step towards removing `struct comedi_file_info`.

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>