b43: Update LP-PHY rev2+ baseband init to match the specs

The rev2+ BB init spec has changed behind us, and thus the code is
no longer up to date. Update the code to match the current specs.
Also implement save/restore dig filt state, as required by the
new specification.

Signed-off-by: Gábor Stefanik <netrolller.3d@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>