b43legacy fixes

These patches are mostly ports from b43. Please note that patch 2/4 is merged with all fixes subsequently posted for the original patch.

It would be fine to have at least patch 4/4 into 2.6.25, but it's not so relevant (only hits users who force DMA mode over PIO with 64 bits processors running a 32 bit kernel with more than 1G of memory - no crash happens though).

John, please tell me if you need me to port this over any other branch. Thanks.

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Ciao
Stefano