Dish Network Corp. Thursday asked regulators to freeze their review of Sprint Nextel Corp.'s $20 billion deal with SoftBank Corp. in light of Dish's offer to buy all or part of mobile broadband company Clearwire Corp. The filing by Dish (Nasdaq: DISH) to the Federal Communications Commission asked that SoftBank’s proposed purchase of a 70 percent stake in Sprint be taken off the usual review deadlines and its approval put on hold. Sprint (NYSE: S) has been seeking spectrum on which to launch…