b43: rewrite A PHY initialization

Rewrite and sync A PHY initialization with specs, thus allowing for further
work to be done on 802.11a support. Note that A PHY initialization involves
G PHYs as well.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Acked-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>