b43: LP-PHY: Implement reading band SPROM

Some of the new variables in b43_phy_lp appear to be dead code in
the vendor driver; they will be removed if they remain unused when
LP-PHY implementation is finished.

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