b43: N-PHY: complete generic support for 0x2057 radio

It doesn't include any device (radio revision) specific code yet, so it
isn't really usable. As the commit says, it's just some generic code.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>