b43: HT-PHY: make it BCMA-only

HT-PHY was found only on BCM4331 which is a BCMA-based chipset. This is
reallly unlikely we will ever see HT-PHY on SSB thus make the whole code
BCMA specific. This will allow us to call various BCMA-specific
functions directly (without extra checks).

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