ISDN: Do not validate ISDN net device address prior to interface-up

Commit bada339b (Validate device addr prior to interface-up) caused a regression
in the ISDN network code, see: http://bugzilla.kernel.org/show_bug.cgi?id=9923


The trivial fix is to remove the pointer to eth_validate_addr() in the
net_device struct in isdn_net_init().

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>