staging: comedi: declare das6402_boards as static

das6402_boards array is used in das6402.c only, so declare it as static.

Signed-off-by: Michele Curti <michele.curti@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>