b43: Add fw capabilities

Add automagic feature flags, so the firmware can tell the driver
about supported features and the driver can switch features on/off as
needed.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Tested-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>