staging: comedi: make comedi_free_board_minor() static

`comedi_free_board_minor()` is now only called from the same .c file it
is defined in, so give it static linkage.

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>