PiFmDma fix for latest wheezy raspbian

Hi Richard,
Thanks much for this excellent code. In the latest raspberry distribution I found that the code is no longer working. After a lot of debugging I finally found out the causes and fixed it. Here are my results. Can you incorporate it in your branch, please?
Guido
PE1NNZ

fix for PiFmDma runtime error in latest 2013-02-09-wheezy-raspbian.
first fix is a type cast for which the sign was not correct
second fix is addition of a bit mask to deal with a reserved bit which now suddenly seems to be 1 (which was assumed to be 0)