b43legacy: fix use-after-free rfkill bug

Fix rfkill code which caused a use-after-free bug. Thanks to David
Woodhouse for spotting this out.

Cc: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>