b43: AC-PHY: prepare place for developing new PHY support

There are new (not anymore?) Broadcom 802.11ac wireless cards based on
chipsets like BCM4352 and BCM4360. They use a new PHY type (called
simply AC) that will require new specific code.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>