MIT is Ok because MIT is compatible with GPL. GPL has language saying you can't add restrictions, but the combination GPL+MIT is essentially GPL so it's ok.
Dual GPL/MIT essentially means that you as a user can choose whether you to use the code as GPL or as MIT, but if you contribute to the code you must provide the full GPL+MIT rights.
As to why release a driver as GPL/MIT instead of just the GPL, I think the idea is that the BSD's (or other OS'es) can take the code and use it under the terms of the MIT license and port it to their kernels. IIRC there are many drivers in Linux that are dual licensed in this way for that reason.
