staging: comedi: refactor comedi_free_board_minor()

Split out the part of `comedi_free_board_minor()` that clears the
`comedi_file_info_table[]` element into new function
`comedi_clear_minor()`.  Split out the remainder of the original
function into new function `comedi_free_board_file_info()`.

Also re-use a call to `comedi_clear_minor()` in
`comedi_free_subdevice_minor()` instead of doing the same thing inline.

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>