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>