b43: Enable LP-PHY support by default and remove Kconfig warning

The most common LP-PHY device, BCM4312, is now fully functional.
So, no need to say "probably won't work for you" anymore.
It's also not "for debuggers and developers only", as it is
perfectly usable for end-users now (at least for BCM4312).

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