b43: fix "‘gmode’ may be used uninitialized" warning

drivers/net/wireless/b43/main.c: In function ‘b43_op_config’:
drivers/net/wireless/b43/main.c:3264: warning: ‘gmode’ may be used uninitialized
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/b43/main.c

@@ -3261,7 +3261,7 @@ static int b43_switch_band(struct b43_wl *wl, struct ieee80211_channel *chan)
 	struct b43_wldev *down_dev;
 	struct b43_wldev *d;
 	int err;
-	bool gmode;
+	bool uninitialized_var(gmode);
 	int prev_status;
 
 	/* Find a device and PHY which supports the band. */