net: b44: set pause params only when interface is up

b44_free_rings() accesses b44::rx_buffers (and ::tx_buffers)
unconditionally, but b44::rx_buffers is only valid when the
device is up (they get allocated in b44_open(), and deallocated
again in b44_close()), any other time these are just a NULL pointers.

So if you try to change the pause params while the network interface
is disabled/administratively down, everything explodes (which likely
netifd tries to do).

Link: openwrt/openwrt#13789
Fixes: 1da177e (Linux-2.6.12-rc2)
Cc: stable@vger.kernel.org
Reported-by: Peter Münster <pm@a16n.net>
Suggested-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Vaclav Svoboda <svoboda@neng.cz>
Tested-by: Peter Münster <pm@a16n.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Peter Münster <pm@a16n.net>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/87y192oolj.fsf@a16n.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/broadcom/b44.c

@@ -2009,12 +2009,14 @@ static int b44_set_pauseparam(struct net_device *dev,
 		bp->flags |= B44_FLAG_TX_PAUSE;
 	else
 		bp->flags &= ~B44_FLAG_TX_PAUSE;
-	if (bp->flags & B44_FLAG_PAUSE_AUTO) {
-		b44_halt(bp);
-		b44_init_rings(bp);
-		b44_init_hw(bp, B44_FULL_RESET);
-	} else {
-		__b44_set_flow_ctrl(bp, bp->flags);
+	if (netif_running(dev)) {
+		if (bp->flags & B44_FLAG_PAUSE_AUTO) {
+			b44_halt(bp);
+			b44_init_rings(bp);
+			b44_init_hw(bp, B44_FULL_RESET);
+		} else {
+			__b44_set_flow_ctrl(bp, bp->flags);
+		}
 	}
 	spin_unlock_irq(&bp->lock);