Use a separate config option for the b43 pci to ssb bridge.

The bridge code was unnecessary enabled by the b44
driver, but it prevents the bcm43xx driver from
being loaded, as the bridge claims the same pci ids.

Now we enable the birdge only if the b43{legacy}
drivers are selected.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>