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>