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: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>