b43: Automatically probe for opensource firmware

First probe for proprietary firmware and then probe for opensource firmware.
This way around it's a win-win situation.
1) If proprietary fw is available, it will work.
2) If opensource firmware is available, but no proprietary (Distros can only ship open fw)
   it might work.
3) If both open and proprietary are available, it will work, because it selects
   the proprietary. We currently don't prefer the open fw in this case, because it doesn't
   work on all devices. It would introduce a regression otherwise.

The remaining FIXMEs in this patch are harmless, because they only matter on multiband
devices, which are not implemented yet anyway.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>