i4l: do not print a warning when shutting down an i4l ppp interface

When an i4l ppp interface is shut down (e.g. with /sbin/ifdown ippp0) a
scary warning is logged:
    
    isdn_free_channel: called with invalid drv(-1) or channel(-1)
    
This warning is caused by isdn_net_unbind_channel(), which always calls
isdn_free_channel() even if isdn_net_local->isdn_device and
isdn_net_local->isdn_channel are (still) in a perfectly acceptable
default state, so let's not do that.
Signed-off-by: NPaul Bolle <pebolle@tiscali.nl>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/isdn/i4l/isdn_net.c

@@ -292,7 +292,9 @@ isdn_net_unbind_channel(isdn_net_local * lp)
 	lp->dialstate = 0;
 	dev->rx_netdev[isdn_dc2minor(lp->isdn_device, lp->isdn_channel)] = NULL;
 	dev->st_netdev[isdn_dc2minor(lp->isdn_device, lp->isdn_channel)] = NULL;
-	isdn_free_channel(lp->isdn_device, lp->isdn_channel, ISDN_USAGE_NET);
+	if (lp->isdn_device != -1 && lp->isdn_channel != -1)
+		isdn_free_channel(lp->isdn_device, lp->isdn_channel,
+				  ISDN_USAGE_NET);
 	lp->flags &= ~ISDN_NET_CONNECTED;
 	lp->isdn_device = -1;
 	lp->isdn_channel = -1;