b43: Don't abuse wl->current_dev in the led work

Don't abuse wl->current_dev in the LED work for checking whether we're
going down. Add an explicit variable.
This fixes a crash on rmmod dereferencing the wl->current_dev NULL pointer
in various other places of the driver.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/b43/leds.h b/drivers/net/wireless/b43/leds.h
index c4a58a0..49d3da5 100644
--- a/drivers/net/wireless/b43/leds.h
+++ b/drivers/net/wireless/b43/leds.h
@@ -35,6 +35,7 @@
 	struct b43_led led_radio;
 	struct b43_led led_assoc;
 
+	bool stop;
 	struct work_struct work;
 };