staging: comedi: das800: move das800_probe()

For aesthetic reasons, move the das800_probe() function near its
only caller, das800_attach().

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>