b43: move hwrng registration driver to wireless core initialization
...and unregistration to core shutdown. Previously, the driver
remained registered even when the hardware was shutdown. That
causes the driver to return -ENODEV if the b43 device is IFF_DOWN.
This change causes the driver to disappear in that case, allowing
/dev/hwrng to still function if another hwrng device is available.
Signed-off-by: John W. Linville <linville@tuxdriver.com>