uml: fix compile error from net_device_ops conversion

Fix the following compile error:

arch/um/drivers/net_kern.c: In function 'uml_inetaddr_event':
arch/um/drivers/net_kern.c:760: error: 'struct net_device' has no member named 'open'

This was introduced by commit 8bb95b39, "uml: convert network device
to netdevice ops".
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jeff Dike <jdike@addtoit.com>
Reviewed-by: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/net_kern.c

@@ -757,7 +757,7 @@ static int uml_inetaddr_event(struct notifier_block *this, unsigned long event,
 	void (*proc)(unsigned char *, unsigned char *, void *);
 	unsigned char addr_buf[4], netmask_buf[4];
 
-	if (dev->open != uml_net_open)
+	if (dev->netdev_ops->ndo_open != uml_net_open)
 		return NOTIFY_DONE;
 
 	lp = netdev_priv(dev);