staging: comedi: fix spelling mistake "desination" -> "destination"

There is a spelling mistake in message text in the call to unittest,
fix this.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>