b43: enable 5 GHz support for N-PHY devices

This has been tested on 14e4:4328 (BCM4321), 14e4:432b (BCM4322),
14e4:4353 (BCM43224) and 14e4:4359 (BCM43228) which is an almost
complete list of 5 GHz capable device (only BCM43222 is missing).

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