b43: add an option to register BROKEN bcma driver

It does nothing useful yet, so it is matched as BROKEN.
For now this is just an option for b43, in future we may want to make
b43 support SSB or BCMA (note: or, not xor).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>