b43: do not call ieee80211_unregister_hw if we are not registred

this patch fixes kernel Oops on "rmmod b43" if firmware was not loaded:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000088
IP: [<ffffffff8104e988>] drain_workqueue+0x25/0x142
PGD 153ac6067 PUD 153b82067 PMD 0
Oops: 0000 [#1] SMP

Signed-off-by: Oleksij Rempel <bug-track@fisher-privat.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/b43/b43.h b/drivers/net/wireless/b43/b43.h
index 67c13af..c06b6cb 100644
--- a/drivers/net/wireless/b43/b43.h
+++ b/drivers/net/wireless/b43/b43.h
@@ -877,6 +877,10 @@
 	 * from the mac80211 subsystem. */
 	u16 mac80211_initially_registered_queues;
 
+	/* Set this if we call ieee80211_register_hw() and check if we call
+	 * ieee80211_unregister_hw(). */
+	bool hw_registred;
+
 	/* We can only have one operating interface (802.11 core)
 	 * at a time. General information about this interface follows.
 	 */