b43: LP-PHY: Remove BROKEN from B43_PHY_LP

Larry has reported success getting scan data with an LP-PHY device,
so it's probably time to release LP-PHY support for testing.

Also disable 802.11a support for now, as 802.11a currently causes
the driver to panic on startup (NULL pointer dereference).

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